kbuild: remove deprecated host-progs variable

The host-progs has been kept as an alias of hostprogs-y for a long time
(at least since the beginning of Git era), with the clear prompt:
  Usage of host-progs is deprecated. Please replace with hostprogs-y!

Enough time for the migration has passed.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>