action_sheet: Remove backward-compat code for unsupported "mark as unread"

Fixes-partly: zulip#1837

Author: chrisbobbe

lib/widgets/action_sheet.dart

@@ -1050,8 +1050,6 @@ void showMessageActionSheet({required BuildContext context, required Message mes
   final isComposeBoxOffered = messageListPage.composeBoxState != null;
 
   final isMessageRead = message.flags.contains(MessageFlag.read);
-  final markAsUnreadSupported = store.zulipFeatureLevel >= 155; // TODO(server-6)
-  final showMarkAsUnreadButton = markAsUnreadSupported && isMessageRead;
 
   final isSenderMuted = store.isUserMuted(message.senderId);
 
@@ -1066,7 +1064,7 @@ void showMessageActionSheet({required BuildContext context, required Message mes
       StarButton(message: message, pageContext: pageContext),
       if (isComposeBoxOffered)
         QuoteAndReplyButton(message: message, pageContext: pageContext),
-      if (showMarkAsUnreadButton)
+      if (isMessageRead)
         MarkAsUnreadButton(message: message, pageContext: pageContext),
       if (isSenderMuted)
         // The message must have been revealed in order to open this action sheet.

lib/widgets/actions.dart

@@ -66,7 +66,6 @@ abstract final class ZulipAction {
     Message message,
     Narrow narrow,
   ) async {
-    assert(PerAccountStoreWidget.of(context).zulipFeatureLevel >= 155); // TODO(server-6)
     final zulipLocalizations = ZulipLocalizations.of(context);
     await updateMessageFlagsStartingFromAnchor(
       context: context,