Remove duplicate inline scaffold in 2up renderer

Summary: Ref T2009. Remove the 4 (!!) copies of this code.

Test Plan:

• Added, edited, and removed inline comments in 2up view.
• Stacked a bunch of comments on the same line and saw the JS place them correctly.
• Created an image diff and added, edited and removed inlines on it.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D12000