remove DB-based skips on non-backend tests

remove DB-based skips on non-backend tests

for tests that are testing for SQL compilation only,
there's no need to limit based on DB backend since we aren't
using it.  this avoids the awkward situation where tests like
test_sqlite.py -> test_create_table_with_comment_ignored would
be skipped when running the test suite against sqlite, but would
run just fine when running against any other DB (because those DBs
dont have a comment limitation and the test is just a compilation
test).

Change-Id: Iecd482e8c1034b28649ce5390dac0d0ddea83790