Remove obsolete docs job from neutron-tempest-plugin-jobs

Currently, neutron repo builds docs twice: with the PTI job
openstack-tox-docs which is part of publish-openstack-docs-pti and with
the obsolete job build-openstack-sphinx-docs which is part of
neutron-tempest-plugin-jobs.

The template neutron-tempest-plugin-jobs is only used for this repo and
for neutron.

Fix:
* Remove the obsolete doc building job so that only the new PTI job is used.

* Add the template build-openstack-docs-pti so that the new PTI jobs run
  on this repo.

Change-Id: I7e5cb3a3fc1b6da0f4b335b631fbaa071cf74631
diff --git a/.zuul.yaml b/.zuul.yaml
index 9a5ac25..4e31ba8 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -722,12 +722,10 @@
         - neutron-tempest-plugin-designate-scenario
         - neutron-tempest-plugin-dvr-multinode-scenario
         - neutron-tempest-plugin-scenario-linuxbridge
-        - build-openstack-sphinx-docs
     gate:
       jobs:
         - neutron-tempest-plugin-api
         - neutron-tempest-plugin-scenario-linuxbridge
-        - build-openstack-sphinx-docs
 
 - project-template:
     name: neutron-tempest-plugin-jobs-queens
@@ -767,6 +765,7 @@
 
 - project:
     templates:
+      - build-openstack-docs-pti
       - neutron-tempest-plugin-jobs
       - neutron-tempest-plugin-jobs-queens
       - neutron-tempest-plugin-jobs-rocky