Cleanup tox.ini: Remove obsolete constraints

Remove old and unused constraints settings from tox.ini. Those
have never been used.

Note that the repo has in the past not used constraints in OpenStack
CI, this change keeps the status quo.

For more information about constraints see:
http://lists.openstack.org/pipermail/openstack-dev/2016-August/101474.html

Change-Id: I7eae051b0568adc75f384e67821eb86d1d2768ab

tox.ini

@@ -5,9 +5,7 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command =
-                  constraints: {[testenv:common-constraints]install_command}
-                  pip install -U {opts} {packages}
+install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt