Remove an outdated directory in tox.ini

The directory openstack/common from the exclude list of
flake8 in tox.ini was used to keep codes from oslo-incubator,
but oslo-incubator was retired, so don't use this directory
any more.

Change-Id: If1ea5dc167cfe4a09aad413b9eab0af807ebe603

Author: Huanxuan Ao

tox.ini

@@ -77,7 +77,7 @@ commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasen
 
 [flake8]
 show-source = True
-exclude = .git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg,build,tools
+exclude = .git,.tox,dist,doc,*lib/python*,*egg,build,tools
 # If 'ignore' is not set there are default errors and warnings that are set
 # Doc: http://flake8.readthedocs.org/en/latest/config.html#default
 ignore = __