crypto: ctr - remove unused crypto_ctr_encrypt_walk()

crypto_ctr_encrypt_walk() is no longer used so remove it.

Note that some existing drivers currently rely on the transitive
includes of some other crypto headers so retain those for the time
being.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>