kconfig: remove SYMBOL_CHOICE flag

All symbols except choices have a name.

Previously, choices were allowed to have a name, but commit c83f0209
("kconfig: remove named choice support") eliminated that possibility.

Now, it is easy to distinguish choices from normal symbols; if the name
is NULL, it is a choice.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>