Delete failing test

I don't think this is testing anything useful, and the test code is
exceedingly brittle. It is broken since
34c7e73c1def1312e59ef1f334586ff2f668246e because the test code makes
assumptions about the implementation of PostgreSQLAdapter#active? which
are incorrect after the commit.

I could fix this test but it would be even more brittle (by stubbing the
underlying @connection.connect_poll) and it doesn't test any complex
logic. I conclude that it's not worth it.