kbuild: rpm-pkg: remove unneeded '-f $srctree/Makefile' in spec file

This is unneeded because the Makefile in the output directory wraps
the top-level Makefile in the srctree.

Just run $MAKE irrespective of the build location.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/package/mkspec

@@ -12,7 +12,6 @@
 # how we were called determines which rpms we build and how we build them
 if [ "$1" = prebuilt ]; then
 	S=DEL
-	MAKE="$MAKE -f $srctree/Makefile"
 else
 	S=