[JENKINS-16917] hudson.plugins.downstream_ext.DownstreamTrigger.DescriptorImpl...

[JENKINS-16917] hudson.plugins.downstream_ext.DownstreamTrigger.DescriptorImpl should not be extending BuildTrigger.DescriptorImpl since it does not produce a BuildTrigger from newInstance.
(Generally there is no reason to subclass a Descriptor type which is already an @Extension rather than an explicit abstract supertype, but this is a bit more conservative change than just making it final.)