Remove install unnecessary packages

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

Change-Id: I35a367505b2b423c345b05519e4134113cb66648

Author: maaoyu

tox.ini

@@ -111,7 +111,6 @@ commands =
 [testenv:docs]
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
-  -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -a -E -W -d doc/build/doctrees -b html doc/source doc/build/html