Remove gravatar support

Summary: Fixes T6261. The performance of asking gravatar for these images is horrible and causing lots of people to have issues with the page.

Test Plan: noted how wildly fast the edit profile picture page loaded

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6261

Differential Revision: https://secure.phabricator.com/D10724