Revert "Merge pull request #4328 from guardian/discussion-activity"

This reverts commit 07312df60098c317b83b2eb1c7d2a6552f6b5c83, reversing
changes made to 078379761cabbbea16b06e73082609f38acf84f6.
Merge pull request #4335 from guardian/discussion-activity-revert

Revert "Merge pull request #4328 from guardian/discussion-activity"