Bug #58841 Generalise handling of plugins in MTR mysql-test-run.pl script

Follow-up fix: mtr died if trying to run semisync test w/o the plugin

mysql-test/mysql-test-run.pl

@@ -2051,10 +2051,7 @@ sub environment_setup {
   read_plugin_defs("include/plugin.defs");
 
   # Simplify reference to semisync plugins
-  if ($ENV{'SEMISYNC_MASTER_PLUGIN'})
-  {
   $ENV{'SEMISYNC_PLUGIN_OPT'}= $ENV{'SEMISYNC_MASTER_PLUGIN_OPT'};
-  }
 
   # --------------------------------------------------------------------------
   # Valgrind need to be run with debug libraries otherwise it's almost