remove sonata admin classes and configuration

[ElectricMaxxx]

fix #257

Todo:

• [x] Remove Admin classes
• [x] remove configuration
• [x] remove tests
• [x] remove dependencies
• [x] tests green

[ElectricMaxxx]

@symfony-cmf/contributors ready to review. Integration can be found in: https://github.com/symfony-cmf/sonata-admin-integration-bundle/pull/23