PluginManager: remove plugin path from sys.path

Remove plugin path from sys.path after importing.

If we scan multiple plugin dirs and never remove the path from
sys.path, it is hard to predict from which path python will import
a package.

The chance that the same plugin package is in multiple plugin paths
is high.

gajim/plugins/pluginmanager.py

@@ -589,6 +589,7 @@ class PluginManager(metaclass=Singleton):
                     log.debug('%s : %s' % (module_attr_name,
                         'wrong manifest file. section are required!'))
 
+        sys.path.remove(path)
         return plugins_found
 
     def install_from_zip(self, zip_filename, owerwrite=None):