use $(file …) directly instead of gb_var2file when not used as intermediate

use $(file …) directly instead of gb_var2file when not used as intermediate

i.e. when the temporary file is not uses as a responsefile for another
command, but instead the point of running gb_var2file is to have that
file as a result in the end.

also move the helper into the main gbuild.mk and remove Tempfile.mk

Change-Id: I79382bef5d7e8a155ba6a8e0750c6a17da42eb37
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/143912
Tested-by: Jenkins
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>