kconfig: Remove the C Kconfig implementation

Remove the C Kconfig tools and various scripts associated with them.

scripts/kconfig/diffconfig is popular, so keep it.

I don't know whether anyone is using scripts/kconfig/config. Remove it
and see if anyone screams.

scripts/kconfig/streamline_config.pl deals with modules ('m' values) and
can safely be removed. Zephyr's Kconfig files do not use modules.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>