Problem:
The warning 1681 "'@@SESSION.GTID_EXECUTED' is deprecated
and will be removed in a future release." was printed even
when the session variable gtid_executed was not included in
the result of the query.
In addition, the query "SELECT @@SESSION.GTID_EXECUTED"
result included a duplicate warning.
Both issues occurred because the warning was printed when
the value of the variable was accessed, which will happen
whether the variable is included in the result of the query
or not.

Fix:
To be consistent with the current behaviour of the deprecated
variable @@GLOBAL.SQL_LOG_BIN, the following changes were
implemented concerning @@SESSION.GTID_EXECUTED:
- the variable is no longer included in the P_S table, therefore
it is not included in the result of:
  a) SELECT * FROM performance_schema.session_variables
  b) SHOW VARIABLES (when SHOW_COMPATIBILITY_56= OFF)
  c) SHOW SESSION VARIABLES (when SHOW_COMPATIBILITY_56= OFF)
- The variable is still a part of the I_S table, but the warning
is not issued in the following statements:
  a) SELECT * FROM information_schema.session_variables
(when SHOW_COMPATIBILITY_56= ON)
  b) SHOW VARIABLES (when SHOW_COMPATIBILITY_56= ON)
  c) SHOW SESSION VARIABLES (when SHOW_COMPATIBILITY_56= ON)
- the above is true even in statements like
SHOW VARIABLES LIKE 'REGEX', even when 'REGEX' matches
'gtid_executed'.
The warning is still issued when the value of the variable is
directly accessed.