[Mnemonics] setLabelFor(null) doesn't remove label binding to the button

To reproduce on Windows:
1. Check "label1.setLabelFor(button1)";
2. Check "label1.mnemonicsParse()";
3. Push ALT and then 1;
Result: focus will be on the Button 1
4. Uncheck "label1.setLabelFor(button1)";
5. Push TAB to remove focus from the Button 1;
6. Push 1.
It is expected that the focus won't move to the Button 1 but it will.