Remove requirements.txt from tox.ini

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

Change-Id: Iaa6026a504cc53784aad5731e9afe8b684b3ede5

tox.ini

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