Remove install unnecessary packages

The docs requirements migrated to doc/requirements.txt
we need not install things from requirements.txt.

Change-Id: Ia65109bbb683489ba9a880d3a41baca5ef560d77

tox.ini

@@ -26,8 +26,7 @@ commands = {posargs}
 
 [testenv:docs]
 deps =
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
 whitelist_externals = rm
 commands =