Remove references to removed $fields parameter

The `get_post()` and `prepare_post()` methods apparently had a
`$fields` parameter at one time. This has been replaced by the
`$context` parameter.

When the `$fields` parameter was removed, some of the inline docs
stayed behind. This removes them.

Incidentally, the inline docs could probably use lots of tweaks. I assume maybe some of this is being put off while the code is still churning.