Cleanup Service descriptor for SearchService

[ignasi35]

As mentioned in https://stackoverflow.com/questions/51262399/lagom-withpathparamserializer, the descriptor for Search Service uses a path param serializer which is unnecessary (https://github.com/lagom/online-auction-java/blob/master/search-api/src/main/java/com/example/auction/search/api/SearchService.java#L23).