Removed image document

This PR can be merged if the image document is implemented and merged on the CmfMediaBundle.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | na
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/191

**TODO to merge**:
- [x] check if all image traces are removed
- [x] update changelog