Due to the removal of the blkcipher and ablkcipher algorithm types,
crypto_skcipher::decrypt is now redundant since it always equals
crypto_skcipher_alg(tfm)->decrypt.

Remove it and update crypto_skcipher_decrypt() accordingly.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>