Remove obscure channel layouts

We still haven't reached a satisfactory solution for audio
configuration, but at least these can go. See #37.

resources/settings/settings_description.json

@@ -73,11 +73,7 @@
         "default": "2.0",
         "possible_values": [
           [ "auto", "Auto Select Channels" ],
-          [ "1.0", "Mono" ],
           [ "2.0", "Stereo" ],
-          [ "2.1", "2.1" ],
-          [ "3.0", "3.0" ],
-          [ "3.1", "3.1" ],
           [ "5.1", "5.1" ],
           [ "7.1", "7.1" ]
         ]