Remove a redundant wait_for_backup_deletion()

The rest_client module has a basic function wait_for_resource_deletion()
and it is enough to implement a small function is_resource_deleted()
in each service client for the same function.
This patch removes the wait_for_backup_deletion() and implement the
is_resource_deleted() for the code cleanup.

Partially implements blueprint consistent-service-method-names

Change-Id: I7c07f1cdde86be850f50825db01eb35c62497820
2 files changed