feat(breaking): Remove PUT/DELETE endpoints for DialogElements (#844)

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## Description
Prep. for model change, DialogElements/Transmissions will be immutable
Removing PUT/DELETE REST endpoints

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## Related Issue(s)

- #836

## Verification

- [x] **Your** code builds clean without any errors or warnings
- [x] Manual testing done (required)
- [x] Relevant automated test added (if you find this hard, leave it and
we'll help out)

## Documentation

- [ ] Documentation is updated (either in `docs`-directory, Altinnpedia
or a separate linked PR in
[altinn-studio-docs.](https://github.com/Altinn/altinn-studio-docs), if
applicable)