crypto: null - Get rid of crypto_{get,put}_default_null_skcipher2()

Since commit 499a66e6

 ("crypto: null - Remove default null
blkcipher"), crypto_get_default_null_skcipher2() and
crypto_put_default_null_skcipher2() are the same as their non-2
equivalents.  So switch callers of the "2" versions over to the original
versions and remove the "2" versions.

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