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/l10n/app_localizations_it.dart

@@ -1097,9 +1097,6 @@ class AppLocalizationsIt extends AppLocalizations {
     return 'Canale $index';
   }
 
-  @override
-  String get channels_hashtagChannel => 'Canale hashtag';
-
   @override
   String get channels_public => 'Pubblico';
 
@@ -1111,12 +1108,6 @@ class AppLocalizationsIt extends AppLocalizations {
   @override
   String get channels_private => 'Privato';
 
-  @override
-  String get channels_publicChannel => 'Canale pubblico';
-
-  @override
-  String get channels_privateChannel => 'Canale privato';
-
   @override
   String get channels_editChannel => 'Modifica canale';