WITH READ LOCK and FLUSH TABLES <list> WITH READ LOCK are
incompatible", which adds information about waits caused by 
FLUSH TABLES statement to deadlock detector in MDL subsystem.

Remove API supporting caching of pointers to TABLE_SHARE 
object in MDL subsystem and all code related to it. 

The problem was that locking requirements of code 
implementing this API conflicted with locking requirements 
of code which adds information about waits caused by flushes 
to deadlock detector in MDL subsystem (the former needed to
lock LOCK_open or its future equivalent while having 
write-lock on MDL_lock's rwlock, and the latter needs to be 
able to read-lock MDL_lock rwlock while owning LOCK_open or 
its future equivalent).

Since caching of pointers to TABLE_SHARE objects in MDL 
subsystem didn't bring expected performance benefits we 
decided to remove caching API rather than try to come up 
with some complex solution for this problem.