Remove V2 API version of coverage extensions

As discussed at the summit in the QA stream, this removes
the coverage extension from the V2 API.

According to the API Change Guidelines
https://wiki.openstack.org/wiki/APIChangeGuidelines
we should not be doing this. However the coverage
extension is a rather special case. It has security issues
such that it shouldn't be used in a production cloud
environment. It also doesn't work properly so its usefulness
to measure API coverage is limited. And in the past the presence of
this plugin has been used as a reason not to accept
an alternative technique to measure API coverage.

This is the proposed replacement

https://review.openstack.org/#/c/25882/

Change-Id: I07d798129ee277a6f7691c25f88c07a5204c0943
Merge "Remove V2 API version of coverage extensions"