Revert "Dequeue changes if they have no hope of merging."

This reverts commit dd26978d5da1e37f75e7fc38201c0d5427417d72.

The performance of mergability checking is poor and starves zuul from
processing other things. revert the change until we can come up with a
better performing solution to this problem.

Change-Id: I744031159572236f61b3fca45bb22492135ebcb2
Reviewed-on: https://review.openstack.org/22251
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: James E. Blair <corvus@inaugust.com>