Hide the Differential scroll objective list on trackpad systems

Summary:
Ref T12733. In the longer run I'd like to just push this out from the edge, but that currently gets us into trouble since we start bumping into content. On my system, the trackpad scrollbar also expands in size when moused over, so the minimum number of pixels we need to push it out is approximatley 15px. This hits body content and the persistent chat.

For now, just disable this element on trackpad systems.

Test Plan:
Disconnected all USB peripherals, quit and relaunched Safari, saw no objective list.

Reconnected mouse, relaunched Safari, saw objective list.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12733

Differential Revision: https://secure.phabricator.com/D17974