Bug#49492 rpl_semi_sync failed on PB2

After stopped slave, it is possible that the Dump thread on master
is still running and has locked the semi-sync master plugin, and when
uninstalling the semi-sync master plugin, a plugin busy warning could
be generated.

Fixed by disabling the warnings when uninstalling semi-sync plugin
on master.

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

@@ -602,7 +602,11 @@ source include/stop_slave.inc;
 UNINSTALL PLUGIN rpl_semi_sync_slave;
 
 connection master;
+# The dump thread may still be running on the master, and so the following
+# UNINSTALL could generate a warning about the plugin is busy.
+disable_warnings;
 UNINSTALL PLUGIN rpl_semi_sync_master;
+enable_warnings;
 
 connection slave;
 source include/start_slave.inc;