Stop specifying requirements.txt in tox.ini

As PBR already injects the contents of requirements.txt into
setup_requires, there is no need for tox to explicitly install them.

http://lists.openstack.org/pipermail/openstack-dev/2015-July/069663.html

Change-Id: I1d9dd87cb1eb08c8a2bcb9cf0d79d90317b079d6

tox.ini

@@ -8,8 +8,7 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/requirements.txt
+deps = -r{toxinidir}/test-requirements.txt
-       -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]
 commands =