MINOR: pattern: introduce pat_ref_delete_by_ptr() to delete a valid reference

MINOR: pattern: introduce pat_ref_delete_by_ptr() to delete a valid reference

Till now the only way to remove a known reference was via
pat_ref_delete_by_id() which scans the whole list to find a matching pointer.
Let's add pat_ref_delete_by_ptr() which takes a valid pointer. It can be
called by the function above after the pointer is found, and can also be
used to roll back a failed insertion much more efficiently.