lock_discard_for_index(): New function, to discard locks for an
index whose index tree has been purged. By definition, such indexes
must be ones for which the MDL upgrade failed in inplace ALTER TABLE
and the ADD INDEX operation was never committed.
Note: Because we do not support online ADD SPATIAL INDEX, we only
have to traverse the lock_sys.rec_hash for B-trees and not the
hash tables for R-trees.

row_purge_remove_clust_if_poss_low(): Invoke lock_discard_for_index()
if necessary before dropping a B-tree for a SYS_INDEXES record.