qt: Don't save geometry for options and about/help window

These dialogs will be something that people occasionally open, not keep
open during their session, so just popping it up in a sensible place
is good enough. Remembering only creates potential issues, like spawning
it outside the current screen area.

On Ubuntu this causes the dialogs to be positioned in the
middle of the main dialog, so I didn't add code for that. YMMV.

Inspired by github pull #5777 by @L-Cranston-Shadow