feature_flags_SUITE: Just set the extra plugin path

rabbit_ct_broker_helpers will take care of prepending the regular
plugins path.

(cherry picked from commit b772a9c)

Author: dumbbell

test/feature_flags_SUITE.erl

@@ -877,13 +877,7 @@ activating_plugin_with_new_ff_enabled(Config) ->
 
 build_my_plugin(Config) ->
     PluginSrcDir = filename:join(?config(data_dir, Config), "my_plugin"),
-    PluginsDir1 = filename:join(?config(current_srcdir, Config), "plugins"),
-    PluginsDir2 = filename:join(PluginSrcDir, "plugins"),
-    PathSep = case os:type() of
-                  {win32, _} -> ";";
-                  _          -> ":"
-              end,
-    PluginsDir = PluginsDir1 ++ PathSep ++ PluginsDir2,
+    PluginsDir = filename:join(PluginSrcDir, "plugins"),
     Config1 = rabbit_ct_helpers:set_config(Config,
                                            [{rmq_plugins_dir, PluginsDir}]),
     case filelib:wildcard("plugins/my_plugin-*", PluginSrcDir) of