Remove trigger-readthedocs job

This job is no longer useful as RTD requires an authenticated
end-point now.  This has been replaced by the trigger-readthedocs role
in I651efdb093df85cea3ab2eaf1a5a9256c87a2ca4.  The dependent change
removes this job from the docs-on-readthedocs template, so it is now
unused.

Change-Id: I794ab68967e8d31d5522acc7484a11d3ebc88ee9
Depends-On: https://review.openstack.org/583834

playbooks/python/readthedocs.yaml

@@ -1,7 +0,0 @@
-- hosts: localhost
-  tasks:
-    - name: Trigger readthedocs build URI
-      uri:
-        method: POST
-        url: 'https://readthedocs.org/build/{{ zuul.project.short_name }}'
-        follow_redirects: all

zuul.yaml

@@ -297,13 +297,6 @@
       - doc/requirements.txt
       - test-requirements.txt
 
-- job:
-    name: trigger-readthedocs
-    description: Send a trigger to the readthedocs url to tell it to build docs
-    nodeset:
-      nodes: []
-    run: playbooks/python/readthedocs.yaml
-
 - job:
     name: multinode
     abstract: true