protocol/bc: remove SigHashOld
protocol/bc: remove SigHashOld

`SigHashOld` isn't used anywhere in Core, and it's not part of the spec: 
https://github.com/chain/cp1/blob/main/spec.md#transaction-signature-hash-type 

Closes chain/chainprv#1614.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@bobg




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain