kbuild: remove redundant clean-files from scripts/kconfig/Makefile

Now mconf, qconf, gconf, nconf are always added to hostprogs-y.
Files added to hostprogs-y are removed by "make clean".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>