QClipboard: remove a useless default switch-case label

The switch condition expression has already dealt width all the
conditions, so a `default` label is not necessary (it introduces a
warning). Delete it.

Fixes: QTBUG-95223
Change-Id: Iac3d9bb5d0b6fd8372d4dc16d920ce0a7b0511db
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/gui/kernel/qclipboard.cpp

@@ -582,9 +582,8 @@ void QClipboard::emitChanged(Mode mode)
         case FindBuffer:
             emit findBufferChanged();
         break;
-        default:
-        break;
     }
+
     emit changed(mode);
 }