[mpris2] Remove unused imports

mpris2_support/mpris2_support.py

 # -*- coding: utf-8 -*-
 
-
 import os
 
-from common import gajim
 from plugins import GajimPlugin
 from plugins.helpers import log_calls
-from plugins.plugin import GajimPluginException
 from common import dbus_support
 
 ERR_MSG = ''