In order to make multi-delete SP friendly we need to have all table 
locks for the elements of main statement table list properly set 
at the end of parsing.

Also performed small cleanup: We don't need relink_tables_for_multidelete()
any longer since the only case now when TABLE_LIST::correspondent_table
is non-zero are tables in auxilary table list of multi-delete and these
tables are handled specially in mysql_multi_delete_prepare().