Removes os-simple-tenant-usage from the V3 API

The extension pre-dates ceilometer and equivalent
functionality is now much better managed using
ceilometer. So we're removing this before the V3 API
is officially released.

Partially implements blueprint v3-api-remove-extensions

Change-Id: Ia8fd475e6f0c4dbc67f3c47279cbaa2280b3af0b
Merge "Removes os-simple-tenant-usage from the V3 API"