Restore skiping rpl.rpl_mdev6020 under Valgrind

(Revert a change done by mistake when XtraDB was removed.)
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

mysql-test/suite/rpl/t/rpl_mdev6020.test

+# Test applies a large binlog, takes long under Valgrind with little benefit.
+--source include/not_valgrind.inc
 --source include/have_innodb.inc
 --source include/have_partition.inc
 --source include/have_binlog_format_mixed_or_row.inc