kconfig: nuke second argument of conf_write_symbol()

Replacing S_TRISTATE by S_BOOLEAN is a no-op for conf_write_symbol().

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
[mmarek: Fix unused variable warning in conf_write()]
Signed-off-by: Michal Marek <mmarek@suse.cz>