...it is only ever called:

* either from dtors, where the virtual-ness doesn't acutally help (and would
  trigger loplugin:fragiledestructor if that were enabled)

* or, in module filter, from executeDialog members of (final) classes directly
  derived from OGenericUnODialog without overriding destroyDialog

Change-Id: If9a7370385a27952189423e5632b3f01e18ddbb5