kbuild: deb-pkg: remove unneeded '-f $srctree/Makefile' in debian/rules

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>
Reviewed-by: Nicolas Schier <n.schier@avm.de>