minor clean up of code and documentation after @simpleEnum annotation PR

[googley42]

the @simpleEnum annotation PR #284 simplified the code and UX for this use case by being more opinionated (removing the choice of encodings)

However we need to:

1. update the codec annotations doc to a) reflect the new lack of choice for the all object case and to explain the default encoding (we can use same attribute map example for the old annotation). We could mention that internally we use the new annotation and that it it automatically added by the ZIO Schema macro.
2. remove the new annotation from the code

[googley42]

closed by #338