cmdlib: Remove acquired_locks attribute from LUs

The “acquired_locks” attribute in LUs is used to keep a list of acquired
locks at each lock level. This information is already known in the lock
manager, which also happens to be the authoritative source. Removing the
attribute and directly talking to the lock manager saves us from having
to maintain the duplicate information when releasing locks.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>