Cocoa: Clear the shortcut used when hiding a native menu entry

The shortcut needs to be cleared if the native menu entry is being
hidden due to the fact it was changed. Otherwise it will not show the
shortcut anymore as it sees it as in-use.

Change-Id: Ifb10db855766e4de71db06ea006f6d63497f3193
Fixes: QTBUG-74113
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/cocoa/qcocoamenu.mm

@@ -250,6 +250,9 @@ void QCocoaMenu::syncMenuItem_helper(QPlatformMenuItem *menuItem, bool menubarUp
             if (wasMerged) {
                 oldItem.enabled = NO;
                 oldItem.hidden = YES;
+                oldItem.keyEquivalent = @"";
+                oldItem.keyEquivalentModifierMask = NSEventModifierFlagCommand;
+
             } else {
                 [m_nativeMenu removeItem:oldItem];
             }