* Revise controllers to use ResponseEntity
* Drop AC xApp control/config screen from front-end
* Drop A1 Mediator controller and tests from back-end
* Drop ENDC and X2 setup requests to E2 Manager
* Upgrade to Spring-Boot 2.2.4.RELEASE
* Set the first instance as the default one
* Revise controllers to use ResponseEntity
* Drop AC xApp control/config screen from front-end
* Drop A1 Mediator controller and tests from back-end
* Drop ENDC and X2 setup requests to E2 Manager
* Upgrade to Spring-Boot 2.2.4.RELEASE
* Set the first instance as the default one