Strip timestamps from popup notification bubbles

Summary:
Fixes T11097. Currently, popup notifications show a useless timestamp with the current time, after D16041 made some things more consistent.

Strip these from the popup bubbles.

Test Plan:

• Saw a popup bubble, no timestamp.
• Viewed main notification list, saw timestamps.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11097

Differential Revision: https://secure.phabricator.com/D16258