[Server] Rework UMLDIOperationHandlerRegistry

[CamilleLetavernier]

As mentioned in this class, this was implemented as an alternative to DIOperationHandlerRegistry, which required to have 1 CreateOperationHandler per Element Type to create. DIOperationHandlerRegistry doesn't have this restriction anymore, so we should be able to use it in a standard way, and remove our custom UMLDIOperationHandlerRegistry