Remove used_limits extension from the V3 API

Remove the used_limits extension from the V3 API because
it shows usage for limits which are no longer going to be displayed
by the limits extension. See https://review.openstack.org/#/c/39872/
The limits are not being displayed there because they are already
available through the quotas extension.

Although the quotas extension currently only displays the quota limit
and not usage, the following proposed changeset adds display
of usage: https://review.openstack.org/#/c/32303/. And this is done
in a way which is much more consistent in terms of naming and interface
than used_limits does.

So we no longer need the used_limits extension

Partially implements blueprint nova-v3-api

Change-Id: I8ec1f300335ca4fc3b86323cc3af6bc1591da7bb
Merge "Remove used_limits extension from the V3 API"