iOS: remove registered undo/redo actions upon qiostextresponder dealloc

QIOSTextResponder add actions to its own NSUndoManager. But
we need to remove those actions again when the responder is
deallocated, otherwise it can lead to a crash in UIKit when
swiping between apps.

Fixes: QTBUG-123843
Pick-to: 6.5 6.2
Change-Id: I404751bc50692a960e568ff2eb0f1754da0cec31
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 42f29da7a5b6c6969c169e9a868696a968ca930d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 9db6df1a56e79c13f942e29a28a607284bcd20ca)

src/plugins/platforms/ios/qiostextresponder.mm

@@ -424,6 +424,7 @@
 {
     self.inputView = 0;
     self.inputAccessoryView = 0;
+    [self.undoManager removeAllActions];
 
     [super dealloc];
 }