Do away with pool._refs

Do away with pool._refs

This collection was added only for the benefit of unit tests
and is unnecessary for the pool to function.  As SQLAlchemy 2.0
will be removing the automatic handling of connections that are
garbage collection, remove this collection so that we ultimately
don't need a weakref handler to do anything within the pool.
The handler will do nothing other than emit a warning that
a connection was dereferenced without being explicitly returned
to the pool, invalidated, or detached.

Change-Id: I4ca196270d5714efbac44dbf6f034e8c7f0af58a