Don't publish Differential "create" action to Asana

Summary: Ref T2852. Currently there's effectively a double notification: one for creating the task, and one for the "alincoln created this revision" story. Drop the "create" story.

Test Plan: Used `bin/feed republish` to republish "create" and non-"create" stories. Verified "create" was dropped as unsupported, while non-"create" went through.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6589