Remove the unused variable trx_t::support_xa.

Also, merge some test changes from MySQL 5.7.10, for deprecating
innodb_support_xa.

mysql-test/suite/sys_vars/r/innodb_support_xa_func.result

@@ -85,4 +85,6 @@ xa rollback 'testb',0x2030405060,11;
 SELECT * from t1;
 a
 30
+disconnect con1;
+connection default;
 DROP table t1;

mysql-test/suite/sys_vars/t/innodb_support_xa_func.test

@@ -20,6 +20,7 @@
 ###############################################################################
 
 --source include/have_innodb.inc
+--source include/count_sessions.inc
 --echo '#--------------------FN_DYNVARS_046_01-------------------------#'
 ####################################################################
 # Check if setting innodb_support_xa is changed in new connection  #
@@ -123,7 +124,10 @@ xa rollback 'testb',0x2030405060,11;
 
 SELECT * from t1;
 
+disconnect con1;
+connection default;
 DROP table t1;
+--source include/wait_until_count_sessions.inc
 
 ########################################################
 # End of functionality Testing for innodb_support_xa   #

storage/innobase/include/trx0trx.h

@@ -1043,7 +1043,6 @@ struct trx_t {
 					for secondary indexes when we decide
 					if we can use the insert buffer for
 					them, we set this FALSE */
-	bool		support_xa;	/*!< normally we do the XA two-phase */
 	bool		flush_log_later;/* In 2PC, we hold the
 					prepare_commit mutex across
 					both phases. In that case, we

storage/innobase/trx/trx0trx.cc

@@ -145,8 +145,6 @@ trx_init(
 
 	trx->check_unique_secondary = true;
 
-	trx->support_xa = true;
-
 	trx->lock.n_rec_locks = 0;
 
 	trx->dict_operation = TRX_DICT_OP_NONE;