crypto: skcipher - remove crypto_skcipher::encrypt

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

Remove it and update crypto_skcipher_encrypt() accordingly.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>