crypto: skcipher - Remove unused crypto_lookup_skcipher() declaration

The definition of crypto_lookup_skcipher() was already removed in
commit 3a01d0ee

 ("crypto: skcipher - Remove top-level givcipher
interface").  So the declaration should be removed too.

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