lib/crypto: sha256: Remove sha256_is_arch_optimized()

Remove sha256_is_arch_optimized(), since it is no longer used.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250630160645.3198-12-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>