Removes the duplicated folder to be watch.

Watching the same folder twice hangs the watcher chokidar. All the plugins
folders are watched and this folder is included by default as plugin folder
and also directly in the watcher configuration.

src/cli/cluster/cluster_manager.js

@@ -86,7 +86,6 @@ module.exports = class ClusterManager {
     const fromRoot = require('../../utils/fromRoot');
 
     this.watcher = chokidar.watch([
-      'src/plugins',
       'src/server',
       'src/ui',
       'src/utils',