Remove linters dependency from content-provider in gate

As part of [1] lets remove the dependency on linters from the
content-provider in the gate, in line with all the other repos.
We don't want to delay the gate for linting since any issues should
be resolved before a change hits the gate.

[1] https://review.opendev.org/q/topic:tripleo-ci-reduce

Change-Id: I31ad623cae3db284b4f6b694033fab5bb9dc4360

zuul.d/layout.yaml

@@ -15,7 +15,7 @@
               - ^molecule-requirements.txt
               - ^requirements.txt
               - ^README.rst
-        - tripleo-ci-centos-8-content-provider: &content-provider
+        - tripleo-ci-centos-8-content-provider:
             dependencies:
               - openstack-tox-linters
         - tripleo-ci-centos-8-scenario004-standalone: &scenario004
@@ -30,7 +30,7 @@
       jobs:
         - openstack-tox-linters
         - openstack-tox-docs: *tripleo-docs
-        - tripleo-ci-centos-8-content-provider: *content-provider
+        - tripleo-ci-centos-8-content-provider
         - tripleo-ci-centos-8-scenario004-standalone: *scenario004
 
     promote: