Remove channel subtitles from UI

Per issue #418, this commmit removes channel subtitles from the channel
list and from the map screen (deep in the marker sharing). This reduces
visual clutter and allows for more compact lists, and the type of
channel is already indicated by the leading icon.

The subtitles simply said "Public channel", "Hashtag channel", or
"Private channel".

We also remove the relevant localization strings.

lib/screens/map_screen.dart

@@ -2011,9 +2011,6 @@ class _MapScreenState extends State<MapScreen> {
                               color: isPublic ? Colors.orange : Colors.blue,
                             ),
                             title: Text(label),
-                            subtitle: isPublic
-                                ? Text(context.l10n.channels_publicChannel)
-                                : null,
                             onTap: () async {
                               Navigator.pop(sheetContext);
                               final canSend = isPublic