kconfig: m/nconf: remove dead code to display value of bool choice

Previously, optional bool choices met the following conditions
simultaneously:

 - sym_is_choice(sym)
 - sym_is_changeable(sym)
 - type == S_BOOLEAN

It no longer occurs since 6a121588 ("kconfig: remove 'optional'
property support"). Remove the dead code.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>