MDEV-17390: re-neable rpl_semi_sync_after_sync test

The reasons sited for disabling this test in MDEV-16172 where disputed.

Author: grooverdan

mysql-test/suite/rpl/disabled.def

@@ -13,6 +13,5 @@
 rpl_partition_archive : MDEV-5077 2013-09-27 svoj Cannot exchange partition with archive table
 rpl_row_binlog_max_cache_size : MDEV-11092
 rpl_row_index_choice : MDEV-11666
-rpl_semi_sync_after_sync : fails after MDEV-16172
 rpl_auto_increment_update_failure : disabled for now
 rpl_current_user : waits for MDEV-22374 fix