Deprecate use of "Signature Target" subpacket in revocation signatures

Introduced in RFC4880, the "Signature Target" subpacket fulfils the following roles:

• In a timestamp or third-party certification signature, it identifies the signature that is being countersigned
• In a revocation signature, it identifies the signature being revoked

But revocation signatures are customarily understood to mean "all previous signatures by me over this material are revoked" (see RFC1991), and only a single revocation signature is required regardless of how many previous signatures were made. Further, revocation signatures are made over the same material as the signature(s) being revoked, so unlike the timestamp or third-party signature types the original signature is neither required for reference, nor a unique identifier. This packet is therefore non-functional when used in a revocation signature, and its use in a revocation signature should therefore be deprecated.