Database tests: remove unused dbtest_fast test database pool

Created by: camdencheek

This was some experimental work I had been playing with to speed up database tests. It was causing issues and I disabled it in our CI. Since then, database migrations have changed dramatically, and if this idea is pursued, it should be started from scratch. At this point though, I have many other things that I'd rather spend my time on, so there is no reason to keep a bunch of stale, unused code lying around.

Test plan

This just removes unused code.