kconfig: remove named choice support

Commit 5a1aa8a1 ("kconfig: add named choice group") did not provide
enough explanation regarding its benefits. A use case was found in
another project [1] sometime later, this feature has never been used in
the kernel.

[1]: https://lore.kernel.org/all/201012150034.01356.yann.morin.1998@anciens.enib.fr/



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>