Remove requirements.txt from tox.ini

From lifeless: pbr reflects the package dependencies from
requirements.txt into the sdist that tox builds.

Change-Id: If54f291e4f3f1a05a1b6142e724cbd33110bac1d

tox.ini

@@ -5,9 +5,7 @@ envlist = py34,py26,py27,pypy,pep8
 distribute = False
 install_command = pip install -U {opts} {packages}
 commands = nosetests -d --with-coverage --cover-inclusive --cover-package cliff []
-deps =
-  -r{toxinidir}/requirements.txt
-  -r{toxinidir}/test-requirements.txt
+deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]
 deps = flake8