fixdep: remove leftover handling of IS_BUILTIN/IS_MODULE

I removed CONFIG_IS_BUILTIN and CONFIG_IS_MODULE in commit
7d78a454 ("linux/kconfig.h: remove unused helper macros"), but
fixdep.c still looks for those. It's harmless, but also pointless and
possibly confusing to a future reader.

Fixes: 7d78a454 ("linux/kconfig.h: remove unused helper macros")
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>