[PD] fix Pad/Pocket logic issue

- fixes the bug reported in https://forum.freecadweb.org/viewtopic.php?f=3&t=70266
  as Method #2
  -> When switching to two dimensions, symmetric must be unchecked if it is checked

src/Mod/PartDesign/Gui/TaskPadParameters.cpp

@@ -107,6 +107,9 @@ void TaskPadParameters::onModeChanged(int index)
         break;
     case Modes::TwoDimensions:
         pcPad->Type.setValue("TwoLengths");
+        // symmetric is then not possible
+        if (ui->checkBoxMidplane->isChecked())
+            ui->checkBoxMidplane->setChecked(false);
         break;
     }