Remove diablo/essex branches.

Those branches are closed, so remove all special configuration:

 * ACLs
 * Bitrot jobs
 * Manuals jobs (which weren't actually being triggered anyway)
 * Zuul branch configuration
 * Zuul job functions.

But leave the now-unused openstack_functions.py file in place
so that it is easy to add in a custom Zuul parameter function
in the future.

Change-Id: Ib0732753543aff0b1de5880fbe8aadc4f8616835
Reviewed-on: https://review.openstack.org/28323
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins