Dead code: commented-out batch endpoint, unused serializer, and its only caller

[abkfenris]

~60 lines of commented-out batch endpoint (including two print() calls), the unused TimeSeriesUpdateResponseSerializer, and a commented sample payload. Git history preserves it. Note TimeSeriesManager.by_dataset_slug (the subject of a separate bug in this batch) exists only to serve this dead endpoint — if batch isn't coming back, both should go together.

Sites: views.py:218-281, serializers.py:198

(from #1780)