deb-pkg: Remove the KBUILD_IMAGE workaround

The arch Makefile are fixed to set KBUILD_IMAGE to the full patch, so
the workaround is no longer needed.

Signed-off-by: Michal Marek <mmarek@suse.com>
Reviewed-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>