[PYG-402] 😕Consistent returning on instanceType (#494)

# Description

**Context**: Pygen creates `pydantic` classes for all views in a given
data model. These comes as Write/Read DTO (data transfer object) classes
for the request and the response. They have a different configurations,
the read/response objects allow for extra properties, while the
write/requests do not. This is in case new properties are added to the
view, the DTO class should not fail when retrieving.

This is causing an issue when the user call
`.list(retrieve_connections="full")`. Pygen builds on top of the
`cognite-sdk` .query method for instances. As part of the instantiation
of the DTO class the nodes\edges are dumped to a dictionary. This dump
includes a `instanceType` property. Since the read/response object
accepts extra properties, this instanceType property gets set on the
resulting DTO item. This is unexpected for the end-user.

This PR removes that `instanceType` from response objects.

## Bump

- [x] Patch
- [ ] Minor
- [ ] Skip

## Changelog
### Fixed

- When calling the `.list` method, the returning items no longer have
`instanceType` attribute set when
`retrieve_connections='full'/'identifier'
