Delete annotations from the resource if they are deleted from the spec

[idlewis]

Currently, the operator doesn't keep track of annotations that have been added to its resources.
If annotations are removed, then it can't work out which annotations have been removed, as some of the existing annotations may have been added by another process (e.g. openshift)
So at the moment, once annotations are added to resources via the RCO spec, those annotations remain there until the whole resource is deleted.