kconfig: remove SYMBOL_NO_WRITE flag

This flag is set to symbols that are not intended to be written
to the .config file.

Since commit b75b0a81

 ("kconfig: change defconfig_list option to
environment variable"), SYMBOL_NO_WRITE is only set to choices.

Therefore, (sym->flags & SYMBOL_NO_WRITE) is equivalent to
sym_is_choice(sym). This flag is no longer necessary.

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