Remove 'Failed to connect to Mythtv backend' popup messages

Currently if the addon is unable to connect to a previously working backend (because backend is suspended or unreachable via network) it displays a pop up every 10 or so seconds, which appears in all menus and over video/visualizations and plays a notification sound. The only way to stop the popups is to disable the addon/livetv.

With the line removed when the backend is unreachable if anything to do with Mythtv is accessed you get the popup with a yellow exclamation mark saying 'No PVR client started yet... Check you configuration or check the logs...'. You still get error messages in the log with the line removed. As soon as the backend is available xbmc connects and live tv functionality is available to use.

https://github.com/fetzerch/xbmc-pvr-addons/issues/110