refactor(ocpp): remove dead parseJsonSchemaFile overrides and standardize validation

refactor(ocpp): remove dead parseJsonSchemaFile overrides and standardize validation

- Delete unused parseJsonSchemaFile overrides from OCPP16/20ServiceUtils
  (createPayloadValidatorMap already passes version via options)
- Remove orphaned JSONSchemaType and JsonType imports
- Standardize null-validator return to false in OCPPRequestService
  (consistent with OCPPIncomingRequestService and OCPPResponseService)
- Change validateRequestPayload and validateIncomingRequestResponsePayload
  visibility from private to protected (consistent with other base classes)