Remove requirements.txt from tox.ini

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

Change-Id: I63548ec321b9c59cc935ba8179b1da5b9c90e09b

tox.ini

@@ -7,8 +7,7 @@ skipdist = True
 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 = ostestr {posargs}
 whitelist_externals = ostestr