Remove non-operational shouldHideFromFeed() from ManiphestTaskPointsTransaction

Summary:
See D18018. Ref T12787. This doesn't actually work; we started publishing these stories as a side effect of converting to ModularTransactions, then I fixed the rendering.

This mechanism has very few callsites and I suspect we may want to get rid of it (see T12787) so just keep publishing these stories for now.

Test Plan: Changed the point value of a task, saw a feed story both before and after the patch.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12787

Differential Revision: https://secure.phabricator.com/D18059