kbuild: drop V=0 support

The top Makefile sets KBUILD_VERBOSE to 0 by default, it looks weird
now because V=1 and V=2 can be OR'ed as V=12. The default should be
empty.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>