Both QNSMenu and QSystemTrayIconQMenu aren't referenced anywhere
else, including within qcocoasystemtrayicon.mm, since the QPA
backend was added.

Change-Id: I632c1b230226b2d08afce7f0f0019e9f7c030ba5
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>