the `headerSoftwareVersion` Byron field is unused and unvalidated

As far as I can tell, the `headerSoftwareVersion` field in a Byron header is not used for anything. (The `headerProtocolVersion`, on the other hand, is used to endorse protocol parameter updates.)

Is it too late to remove this from `ouroboros-consensus-byron`? (And maybe even from `cardano-ledger`?) We'd need to fill the corresponding bytes on the wire to remain binary compatible, but having realistic looking values floating around that serve no purpose and are never checked can only ever cause confusion.

Alternatively, I suggest this field should be fixed/required by the source code to be the corresponding version number, instead of being read unvalidated from a config file etc.