Don't set isThread on the message and file delegate context menus

It doesn't have a property called isThread, and I don't know where it
went - if it ever existed?

(cherry picked from commit 07d3b80c3e)

src/qml/RoomPage.qml

@@ -253,7 +253,6 @@ Kirigami.Page {
                 messageComponentType: messageComponentType,
                 plainText: plainText,
                 htmlText: htmlText,
-                isThread: isThread
             });
             contextMenu.open();
         }
@@ -265,7 +264,6 @@ Kirigami.Page {
                 plainText: plainText,
                 mimeType: mimeType,
                 progressInfo: progressInfo,
-                isThread: isThread
             });
             contextMenu.open();
         }