Removal of redundant encoding definitions in ZIP 227 (#82)

This removes the tables with the encoding of the issuance action
description and issuance bundle from ZIP 227. This same material has
been included in ZIP 230 as part of the V6 transaction format, and thus
it is simpler to have it defined in a single place.

ZIP 227 has been updated with references to the relevant sections of ZIP
230 where necessary.