Don't show remote events in the delta sync API

Summary:
So, Esper have reported that they've got duplicate events in the delta sync output. I turns out that was because of a subtlety of the events sync engine. When we're syncing events, we store two different objects pointing to the same event: a local version (with local changes) and a remote version. The problem is we surface both objects in the delta sync API, which is confusing for our customers.

This patch fixes this, in a myopic way. Going forward, I think we'll have to break up the delta sync API to have one endpoint for each resource. As is, it's pretty complicated to have custom filtering/formatting of the data.

Test Plan: Tested manually. Checked that only local events surfaced.

Reviewers: kav-ya, emfree

Reviewed By: emfree

Subscribers: spang

Projects: #sync-engine, #customers

Maniphest Tasks: T663

Differential Revision: https://review.inboxapp.com/D871