basehub: User release_request_pad instead of remove_pad

Since the element is not self element the correct way is call
release_request_pad, and let the element perform the necessary
steps to release its pad.

Change-Id: I0b258cbf2f6ea1e049e0a1bb74449c08dfddf224
basehub: Remove target pad just before removing the pad

Change-Id: I5b24e7fe5ee718c6767458ce53f0ae7e97cfef0c