Drop the final dependency on the Postgres test_parser extension.
We also have to remove references to test_parser in the migration where
it was first defined, otherwise replaying all migrations from the
beginning will fail. Replaying all migrations from the beginning
normally isn't done except in testing.
After this, it should be possible to use a vanilla install of Postgres
with Danbooru. It's still recommended to use Danbooru's Docker image for
Postgres (https://ghcr.io/danbooru/postgres), as other Postgres extensions
may be necessary in the future.
