Post-fix for MDEV-12050 Remove unused InnoDB Memcached hooks

Remove also the field trx_t::read_write that was only used by
the Memcached hooks.

storage/innobase/include/trx0trx.h

@@ -1261,9 +1261,6 @@ struct trx_t {
 					transaction branch */
 	trx_mod_tables_t mod_tables;	/*!< List of tables that were modified
 					by this transaction */
-        /*------------------------------*/
-	bool		read_write;	/*!< if read and write operation */
-
 	/*------------------------------*/
 	char*		detailed_error;	/*!< detailed error message for last
 					error, or empty. */

storage/innobase/trx/trx0trx.cc

@@ -462,8 +462,6 @@ trx_create_low()
 	/* We just got trx from pool, it should be non locking */
 	ut_ad(trx->will_lock == 0);
 
-	trx->read_write = true;
-
 	/* Background trx should not be forced to rollback,
 	we will unset the flag for user trx. */
 	trx->in_innodb |= TRX_FORCE_ROLLBACK_DISABLE;