This was only used to specify XP as a target which is
not supported on 5.8 anymore. Clean up all associated
special handling in the mkspecs and pro files.
This effectively reverts change 10a0ac75.
Change-Id: I420d73002912989f1a5be961a2d09277ec4a4425
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>