Fixed rpl_innodb_mixed_ddl and rpl_000015.

Respectively, replaced "--exec diff" by "--diff_files" which is a mysqltest command to run a
non-operating system specific diff. Removed the file rpl_000015-slave.sh as it is not
necessary in the new MTR.

mysql-test/suite/rpl/include/rpl_mixed_ddl.inc

@@ -83,4 +83,4 @@ sync_slave_with_master;
 # will be created. You will need to go to the mysql-test dir and diff
 # the files your self to see what is not matching
 
---exec diff $MYSQLTEST_VARDIR/tmp/rpl_switch_stm_row_mixed_master.sql $MYSQLTEST_VARDIR/tmp/rpl_switch_stm_row_mixed_slave.sql
+--diff_files $MYSQLTEST_VARDIR/tmp/rpl_switch_stm_row_mixed_master.sql $MYSQLTEST_VARDIR/tmp/rpl_switch_stm_row_mixed_slave.sql

mysql-test/suite/rpl/t/rpl_000015-slave.sh

-rm -f $MYSQLTEST_VARDIR/slave-data/master.info