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>