Remove Placement API reference

The placement project has published the API reference
in its own repository and the related jobs for the nova project
has been removed since Ia4680f24d78af1260f2f0106a458b78a079c1287.

So remove the files and definitions related to
the placement API reference in the nova repository.

Change-Id: Ia43b958a28e7e763e7ecb29e06f8d21d2b9a850f

tox.ini

@@ -195,15 +195,6 @@ commands =
   rm -rf api-ref/build
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 
-[testenv:placement-api-ref]
-description =
-  Generate the Placement API ref. Called from CI scripts to test and publish to developer.openstack.org.
-envdir = {toxworkdir}/docs
-deps = {[testenv:docs]deps}
-commands =
-  rm -rf placement-api-ref/build
-  sphinx-build -W -b html -d placement-api-ref/build/doctrees placement-api-ref/source placement-api-ref/build/html
-
 [testenv:releasenotes]
 description =
   Generate release notes.
@@ -222,7 +213,6 @@ commands =
   {[testenv:docs]commands}
   {[testenv:api-guide]commands}
   {[testenv:api-ref]commands}
-  {[testenv:placement-api-ref]commands}
   {[testenv:releasenotes]commands}
 
 [testenv:bandit]