Remove mysql_lock_have_duplicate(), since now we always
have TABLE_LIST objects for MyISAMMRG children
in lex->query_tables and keep it till the end of the
statement (sub-statement).


mysql-test/r/merge.result:
  Update results (Bug#54811).
mysql-test/t/merge-big.test:
  Update to new wait state.
mysql-test/t/merge.test:
  Add a test case for Bug#54811.
sql/lock.cc:
  Remove a function that is now unused.
sql/lock.h:
  Remove a function that is now unused.
sql/sql_base.cc:
  Don't try to search for duplicate table among THR_LOCK objects, TABLE_LIST list contains all used tables.