kbuild: deb-pkg: remove firmware package support

Commit 5620a0d1 ("firmware: delete in-kernel firmware") deleted
in-kernel firmware support, including the firmware install command.

So, the firmware package does not make sense any more.  Remove it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Riku Voipio <riku.voipio@linaro.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>