Cut down on some code duplication in delete.py

[jwodder]

Closes #606.

[codecov]

Codecov Report

Merging #610 (a58e164) into master (cf63ff9) will increase coverage by 0.16%.
The diff coverage is 73.33%.

@@            Coverage Diff             @@
##           master     #610      +/-   ##
==========================================
+ Coverage   84.02%   84.18%   +0.16%     
==========================================
  Files          59       59              
  Lines        5728     5711      -17     
==========================================
- Hits         4813     4808       -5     
+ Misses        915      903      -12     

Flag	Coverage Δ
unittests	84.18% <73.33%> (+0.16%)	⬆️

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Impacted Files	Coverage Δ
dandi/delete.py	92.56% <73.33%> (+6.50%)	⬆️

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[yarikoptic]

Thank you! looks legit