Fix trigger drop procedure
After commit
123cc697a8eb, we remove redundant FK action triggers during
partition ATTACH by merely deleting the catalog tuple, but that's wrong:
it should use performDeletion() instead. Repair, and make the comments
more explicit.
Per code review from Tom Lane.
Discussion: https://postgr.es/m/18885.
1549642539@sss.pgh.pa.us