remove cobertura from build file

Since the project has adopted Jacoco, I don't believe the cobertura tasks are in use any longer, and it's not certain if they still function. I don't think there's any benefit from trying to keep both coverage tools working, and also have the impression that cobertura development has slowed (or been slow to support new versions of java). Jacoco's other advantage is it's simpler usage (via a java agent), as compared to cobertura's offline code instrumentation requiring more complexity.