Convert the old feature system

... to the new qmake based configuration system.

This removes the old qfeatures.txt (distributed over configure.json
files) and qfeatures.h (distributed over qconfig-<module>.h files).

qfeatures.prf is gone without replacement, as attempts to use it would
lead to followup errors anyway.

Change-Id: I1598de19db937082283a905b9592d3849d2199d0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>