Fix litmus failing on ocis storage

[butonic]

We now ignore the no data available error when removing a non existing metadata attribute, which is ok because we are trying to delete it anyway.

fixes #1178

[butonic]

docker run -e LITMUS_URL="https://172.17.0.1:9200/remote.php/webdav/" -e LITMUS_USERNAME="einstein" -e LITMUS_PASSWORD="relativity" owncloud/litmus:latest
-> running `basic':
 0. init.................. pass
 1. begin................. pass
 2. options............... pass
 3. put_get............... pass
 4. put_get_utf8_segment.. pass
 5. put_no_parent......... pass
 6. mkcol_over_plain...... pass
 7. delete................ pass
 8. delete_null........... pass
 9. delete_fragment....... pass
10. mkcol................. pass
11. mkcol_again........... pass
12. delete_coll........... pass
13. mkcol_no_parent....... pass
14. mkcol_with_body....... pass
15. finish................ pass
<- summary for `basic': of 16 tests run: 16 passed, 0 failed. 100.0%
-> running `copymove':
 0. init.................. pass
 1. begin................. pass
 2. copy_init............. pass
 3. copy_simple........... pass
 4. copy_overwrite........ pass
 5. copy_nodestcoll....... pass
 6. copy_cleanup.......... pass
 7. copy_coll............. pass
 8. copy_shallow.......... pass
 9. move.................. pass
10. move_coll............. pass
11. move_cleanup.......... pass
12. finish................ pass
<- summary for `copymove': of 13 tests run: 13 passed, 0 failed. 100.0%
-> running `props':
 0. init.................. pass
 1. begin................. pass
 2. propfind_invalid...... pass
 3. propfind_invalid2..... pass
 4. propfind_d0........... pass
 5. propinit.............. pass
 6. propset............... pass
 7. propget............... pass
 8. propextended.......... pass
 9. propmove.............. pass
10. propget............... pass
11. propdeletes........... pass
12. propget............... pass
13. propreplace........... pass
14. propget............... pass
15. propnullns............ pass
16. propget............... pass
17. propremoveset......... pass
18. propget............... pass
19. propsetremove......... pass
20. propget............... pass
21. propvalnspace......... pass
22. propwformed........... pass
23. propinit.............. pass
24. propmanyns............ pass
25. propget............... pass
26. propcleanup........... pass
27. finish................ pass
<- summary for `props': of 28 tests run: 28 passed, 0 failed. 100.0%
-> running `locks':
 0. init.................. pass
 1. begin................. pass
 2. options............... pass
 3. precond............... pass
 4. init_locks............ pass
 5. put................... pass
 6. lock_excl............. pass
 7. discover.............. pass
 8. refresh............... pass
 9. notowner_modify....... FAIL (DELETE of locked resource should fail)
10. notowner_lock......... FAIL (LOCK on locked resource)
11. owner_modify.......... pass
12. notowner_modify....... FAIL (DELETE of locked resource should fail)
13. notowner_lock......... FAIL (LOCK on locked resource)
14. copy.................. FAIL (could not COPY locked resource:
404 Not Found)
15. cond_put.............. SKIPPED
16. fail_cond_put......... SKIPPED
17. cond_put_with_not..... pass
18. cond_put_corrupt_token FAIL (conditional PUT with invalid lock-token should fail: 204 No Content)
19. complex_cond_put...... pass
20. unlock................ FAIL (UNLOCK on `/remote.php/webdav/litmus/lockme': 501 Not Implemented)
21. fail_cond_put_unlocked FAIL (conditional PUT with invalid lock-token should fail: 204 No Content)
22. lock_shared........... FAIL (requested lockscope not satisfied!  got shared, wanted exclusive)
23. notowner_modify....... SKIPPED
24. notowner_lock......... SKIPPED
25. owner_modify.......... SKIPPED
26. double_sharedlock..... SKIPPED
27. notowner_modify....... SKIPPED
28. notowner_lock......... SKIPPED
29. unlock................ SKIPPED
30. prep_collection....... pass
31. lock_collection....... pass
32. owner_modify.......... pass
33. notowner_modify....... FAIL (DELETE of locked resource should fail)
34. refresh............... pass
35. indirect_refresh...... pass
36. unlock................ FAIL (UNLOCK on `/remote.php/webdav/litmus/lockcoll/': 501 Not Implemented)
37. unmapped_lock......... See debug.log for network/debug traces.
Segmentation fault

well, locally it segfaults in the locking tests ... but the props work

[phil-davis]

litmus passes in #1166 in CI with this code. LGTM
After this PR is merged, I can rebase #1166 and that CI can be merged. Then we will know if any of this has a regression in future PRs.

[phil-davis]

@labkode @ishank011 please review