hs-v3: Simplify decrypt_desc_layer interface

Here is how decrypt_desc_layer is called:

  superencrypted_len = decrypt_desc_layer(desc,
                                 desc->plaintext_data.superencrypted_blob,
                                 desc->plaintext_data.superencrypted_blob_size,
                                 NULL, 1, &superencrypted_plaintext);

  encrypted_len = decrypt_desc_layer(desc,
                                 desc->superencrypted_data.encrypted_blob,
                                 desc->superencrypted_data.encrypted_blob_size,
                                 descriptor_cookie, 0, &encrypted_plaintext);

There is no point in passing desc->superencrypted_data.encrypted_blob and desc->superencrypted_data.encrypted_blob_size since we are already passing the whole desc and is_superencrypted_layer which should be enough to figure out which fields to use.

We could either of the following two things:

• Ditch desc as an argument and pass desc->plaintext_data.blinded_pubkey explicitly.
• Ditch encrypted_blob and encrypted_blob_size as arguments and get them off desc.

I prefer the first, but I'm fine with either, since it will make the interface cleaner.