Remove the ability to leave multi-line inline comments on touchscreen devices

Summary: Ref T12733. This ultimately conflicts with scrolling and took about two days to get reported as a bug/regression. See T12733 for a bunch of additional discussion. See T1026 for original discussion.

Test Plan:

• Left single-line and multi-line comments on desktop.
• Tapped to leave single-line comments on mobile.
• Dragged lines on mobile, got a scroll instead of a range comment.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12733

Differential Revision: https://secure.phabricator.com/D18044