Makefile: Remove unnecessary dependencies

The rule included from auto.conf.cmd adds a dependency on every Kconfig
file, so these two Kconfig files do not need to be specified again here.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>