Remove dead helper function from test_l3_plugin

This code is no longer used anywhere, so it
can be safely removed.

Change-Id: I23cc2e2cafda2408c98e052a6667cd303335331f
Revert "VPNaaS REST Client UT Broken"

I8badc249ad021fdbdb2367b5416c72435ed58994 causes anything importing
neutron/tests/unit/services/vpn/device_drivers/_test_cisco_csr_rest.py
to exit since httmock isn't a dependency.

Fix all hacking issues in addition to the revert, as this patch fixes 'tox -epep8'.
And just a reverting the patch will cause pep8 to fail.

Fixes-Bug: #1340881

This reverts commit 7f0a8f09ab7867100081a898347a5f901e5a442b.

Change-Id: I373a8c8ab16eb387be6a451b8146642389081afa
Merge "Remove dead helper function from test_l3_plugin"
Merge "Revert "VPNaaS REST Client UT Broken""