sharedptr_registry: remove extaneous Mutex::Locker declaration

For some reason, the lookup() retry loop (for when happened to
race with a removal and grab an invalid WeakPtr) locked
the lock again.  This causes the #3836 crash since the lock
is already locked.  It's rare since it requires a lookup between
invalidation of the WeakPtr and removal of the WeakPtr entry.

Fixes: #3836
Backport: bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>