crypto: skcipher - remove crypto_skcipher_extsize()

Due to the removal of the blkcipher and ablkcipher algorithm types,
crypto_skcipher_extsize() now simply calls crypto_alg_extsize().  So
remove it and just use crypto_alg_extsize().

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