include: remove unused macros from include/arch/*/arch.h

All architecture defines OCTET_TO_SIZEOFUNIT and SIZEOFUNIT_TO_OCTET
as identity functions. But the only user is tests/benchmarks/app_kernel.
It's effectively a no-op.  Remove them.

Signed-off-by: Yasushi SHOJI <y-shoji@ispace-inc.com>