Remove bogus length checks from EVP_aead_chacha20_poly1305()

Remove bogus length checks from EVP_aead_chacha20_poly1305()

The length checks for EVP_aead_chacha20_poly1305() seal/open were incorrect
and are no longer necessary (not to mention that the comment failed to
match the code). Remove these since the underlying ChaCha implementation
will now handle the same sized inputs at these functions can.

Issue flagged by and ok tb@