kconfig/nconf: remove useless conditionnal

After the test

	if (!submenu || ...)
		continue;

the variable `submenu' can _not_ be NULL, so do not test for this
situation.

Cc: Nir Tzachar <nir.tzachar@gmail.com>
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>