msglist [nfc]: Remove a hard-coded 'en_US' which is the default

See the doc:
  https://pub.dev/documentation/intl/latest/intl/DateFormat-class.html
> Formatting dates in the default 'en_US' format does not require
> any initialization.

(And we haven't been doing the described initialization for 'en_US'
or any other locale; it's asynchronous, and we have a better plan
for international formatting described in #45, using ffi.)

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -2011,7 +2011,7 @@ enum MessageTimestampStyle {
       return DateFormat.yMMMd().format(dateTime);
     }
   }
-  static final _timeOnlyFormat = DateFormat('h:mm aa', 'en_US');
+  static final _timeOnlyFormat = DateFormat('h:mm aa');
   static final _lightboxFormat = DateFormat.yMMMd().add_Hms();
   static final _fullFormat = DateFormat.yMMMd().add_jm();