linux/kconfig.h: remove unused helper macros

U-Boot does not have loadable modules, and nothing currently uses any
of the (CONFIG_)?IS_(BUILTIN|MODULE) macros - only
the (CONFIG_)?IS_ENABLED variants are ever used.

While I understand the desire to keep this somewhat synchronized with
linux, we've already departed by the introduction of the
CONFIG_IS_ENABLED extra logic, and deleting these makes the next patch
much simpler, since I won't have to duplicate a lot of logic for no
real gain (as there are no users).

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>