Revert "Ensure we are not exhausting the sqlalchemy pool"

This reverts commit 64bc862f6fa936dbe5dd2d402240a81da40bc5a5.

Now that we don't monkey patch anymore with eventlet, we're sure the
situation fixed previously by that patch where a process could consume
more messages than having SQL connection can't happen anymore.

Change-Id: Ia0f1037e1b8c3e079d382c608e047614e388ce37