Move qmake files to standard locations

The tools/qmake directory is a left over from webkit where qmake was
but one of many buildsystems. This is not necessary for qtwebengine, so
we can instead use the standard locations.

Change-Id: I3d126d6627295b113b091f3eabeee25f1c1f6183
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>