The variables these files refer to are not set anywhere anymore.
The Unix thread.prf file is still needed, as it is still effective and
is activated by qt.prf.
Task-number: QTBUG-25106
Change-Id: Ia514192d28785205df3710d78ee597285d4136b0
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>