This fixes a regression from Bug#21354712 SHOW VARIABLES RETURN WARNING ABOUT @@SESSION.GTID_EXECUTED BEING DEPRECATED, where the logic to deprecate "gtid_executed" misaligns the system variable cache, causing the position of variables following "gtid_executed" to be off by one.

This patch introduces System_variable::m_ignore to identify deprecated system variables that must be accounted for internally but otherwise ignored.