Remove write-kolla-tag.yml custom playbook & hooks

It is no longer necessary since we generate a tag in advance.

Author: markgoddard

README.rst

@@ -459,10 +459,7 @@ At this point you are ready to build and push some container images.
    kayobe seed container image build --push
    kayobe overcloud container image build --push
 
-The container images are tagged as ``wallaby-<datetime>``. This Kayobe
-configuration includes a hook that writes the tag to ``~/kolla_tag``, since
-it is not always simple to determine which tag was last applied to built
-images.
+The container images are tagged as ``wallaby-<datetime>``.
 
 To use the new images, edit
 ``~/src/kayobe-config/etc/kayobe/kolla.yml`` to set the above