kconfig: remove unneeded menu_is_visible() call in conf_write_defconfig()

When the condition 'sym == NULL' is met, the code will reach the
'next_menu' label regardless of the return value from menu_is_visible().

menu_is_visible() calculates some symbol values as a side-effect, for
instance by calling expr_calc_value(menu->visibility), but all the
symbol values will be calculated eventually.

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