Drop the ownCloud migration table instead of reusing it

[nickvergessen]

Fix #7528

[rullzer]

Sounds sane :)

[informatico-madrid]

Hi. in my case i update from owcloud 8 to nextcloud 9 with script migration. then i update -> 10 -> 11 -> 12 and 13 BETA RC. This cannot remove my migration table because connection try remove database with name : oc_oc_migration. SO i replace this line; 7154050#diff-333567435f8bef9975c29c5e73ab7dd0R132
to this -> $this->connection->dropTable('migrations');
and work for mi now.

[nickvergessen]

Thanks for the hint, no idea how that happened:
#7912