Redundant code in storage_backend API

[junboli]

Brief Description
-----------------
The base code already provide storage_ceph, storage_lvm, storage_file..., and
already returned by bellow code:
   'return _patch(storage_backend_uuid, patch)'
So, after the return code, there is no use and we don't need keep those code again.

Severity
--------
Major

Steps to Reproduce
------------------
Just remove those dead code.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/668583
Committed: https://git.openstack.org/cgit/starlingx/config/commit/?id=0e625db2b13c42aba04e8e9424f1a1aaeacd8886
Submitter: Zuul
Branch: master

commit 0e625db2b13c42aba04e8e9424f1a1aaeacd8886
Author: lijunbo <email address hidden>
Date: Mon Jul 15 23:08:29 2019 -0400

    Cleanup dead code in storage_backend.py

    The base code already provide storage_ceph,
    storage_lvm, storage_file..., and already
    returned by bellow code:
       'return _patch(storage_backend_uuid, patch)'
    So, after the return code, there is no use and
    we don't need keep those code again.

    Closes-bug: #1836678
    Change-Id: Ia83e674688b583a40205da5d26d26de150e326a7
    Signed-off-by: junboli <email address hidden>

[Ghada Khalil (gkhalil)]

Low priority / not gating - code cleanup with no functional impact