kconfig: Drop support for CONFIG_TOOLCHAIN_VARIANT

The Kconfig option TOOLCHAIN_VARIANT (not to be confused with
ZEPHYR_TOOLCHAIN_VARIANT) is a legacy configuration option that has
very few use-cases and can easily be dropped.

It's functionality is easily covered by CONFIG_X86_IAMCU and
ZEPHYR_TOOLCHAIN_VARIANT.

This commit removes all references of it from Zephyr.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>