Remove redundant requirements.txt from tox.

pbr's reflection pulls in all the runtime requirements automatically -
the only reason to have the txt file explicitly listed is to permit
things that can't be reflected like url references - which we've
stopped using anyway.

Change-Id: I81b7ac268d5c007f74a2ab1dce94da41d6d75c7b

tox.ini

@@ -7,8 +7,7 @@ skipsdist = True
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
 install_command = pip install -U {opts} {packages}
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
+deps = -r{toxinidir}/test-requirements.txt
 commands = lockutils-wrapper python setup.py testr --slowest --testr-args='{posargs}'
 whitelist_externals = bash
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY