Removes dashboard item poller when dashboard item is removed.
Removes dashboard item poller when dashboard item is removed.

resolves #460

spring-cloud/spring-cloud-dataflow-ui,ghillert/spring-cloud-dataflow-ui,cppwfs/spring-cloud-dataflow-ui,cppwfs/spring-cloud-dataflow-ui,BoykoAlex/spring-cloud-dataflow-ui,spring-cloud/spring-cloud-dataflow-ui,cppwfs/spring-cloud-dataflow-ui,ghillert/spring-cloud-dataflow-ui,BoykoAlex/spring-cloud-dataflow-ui,BoykoAlex/spring-cloud-dataflow-ui,ghillert/spring-cloud-dataflow-ui,BoykoAlex/spring-cloud-dataflow-ui,ghillert/spring-cloud-dataflow-ui,spring-cloud/spring-cloud-dataflow-ui,spring-cloud/spring-cloud-dataflow-ui,cppwfs/spring-cloud-dataflow-ui