Remove obsolete playbook for placement-api-ref

Change Ia4680f24d78af1260f2f0106a458b78a079c1287 remove the
placement-api-ref job but forgot to remove a playbook - remove this
as cleanup.

Change-Id: I4fb060a7c37a680854ef6f577ab6ede00bf1ff00

playbooks/publish/placement-api-ref.yaml

@@ -1,8 +0,0 @@
-- hosts: all
-  roles:
-    - role: write-root-marker
-      when: zuul_success | bool
-    - role: fetch-sphinx-output
-      sphinx_output_suffix: "/"
-      zuul_executor_dest: "{{ zuul.executor.work_root }}/artifacts"
-      when: zuul_success | bool