Removes support for the variable "cinder_backends_rbd_inuse" since
it has been removed from use in the integrated repo.
Change-Id: I581cc7f53d3d3f9abace5b0294d735a27beb59f6
Depends-On: I6a76e63881150677352520ce9658f1cf6b7e5456
(cherry picked from commit 92c6aa07aa)