[BIP322] remove empty message requirement for full (proof-of-funds) proofs

[ZenulAbidin]

Signed messages must be displayed in

, however for that to be possible, there must be a Message in the first place.

This changes removes those requirements of BIP322 that do away with the Message in Full (proof of funds) proof, thereby allowing it to be used as both a proof-of-address-control and proof-of-UTXO-control signature.

Currently, you need to make two separate proofs to demonstrate control over both the address and for UTXOs, since most verifiers are not full nodes and have no UTXO set.

Note: Contrary to what Github claims, there is only one commit being merged here. The other 10 are related to a file that I have already removed. I will rebase the commits when I get a chance.
Note 2: If these delted parts are referring to the address/scriptPubKey, then this PR can be disregarded, but I strongly believe that the spec should mention that Output Points should be filled in the place of the Address field in a signed message [i.e. txid:n strings].

[kallewoof]

The empty requirement was originally proposed by @luke-jr -- I have no strong opinions on the matter, but I'll give him the opportunity to address the suggestion before making a decision.

[murchandamus]

@kallewoof, @luke-jr: What’s the status of this?

[kallewoof]

ACK