Replace `NoteInterface::pack_content` and `NoteInterface::unpack_content` with standard `Packable`

[benesjan]

Now that NoteHeader is nuked (by this PR) we can use standard Packable trait for notes instead of the custom NoteInterface::pack_content and NoteInterface::unpack_content.

All the mentions of note content should be replace just with a note because now the concept of note content does not make sense - note content is a note.