migration store: Fix golang-migrate schema table name usage

Created by: efritz

The schema name that we supply to the migration store is already the exact table name that our old golang-migrate machinery used. I incorrectly added a transform that attempts to query the schema_migrations_schema_migrations table, which doesn't exist (schema_migrations does, though).

Removing this transform uses the correct table.

Test plan

Tested locally; updated unit tests.