DROP_TABLE_PROC().

With this change I observe a speedup from 6.2s to 0.1s when executing
DROP_TABLE_PROC() during DROP TABLE with 512 foreign keys, like what
is being done in innodb_bug56143.test

This fixes "Bug#11765460 DROP TABLE USES INEFFICIENT METHODS TO REMOVE
FKS/INDEXES FROM INNODB SYS TABLES"

Reviewed by:	Marko