This implements the ability to drop foreign keys
with sqlite. It's basically the same implementation
used for dropping unique constraints so the common
code is refactored.
The existing FKey test that was skipping sqlite is
no longer skipped to show this works.
Change-Id: Idaaf4229e34af4c21c3bcead4b4e22491d24238e
Closes-Bug: #1423955
ae64d828df