Better return values for concept cluster/pipeline deletion method

[fmatthies]

The following two deletion methods both return a ResponseEntity<void>:

• ConceptPipelineApiDelegateImpl.deleteConceptPipelineById
• ConceptClusterApiDelegateImpl.deleteConceptClustersForPipelineId

Maybe it's more feasible to change this to a ResponseEntity<PipelineResponse>; this would need a change in top-api as well, as those methods are overrides.

[fmatthies]

For deleteConceptPipelineById one could also think about an option to only partially delete (or better pick up where failed previously because there are already steps present)
The second option could/should be checked in top-frontend startPipeline.