R1:a33409991c27

Remove an old Differential selection behavior

Summary:
Ref T12822. Ref PHI878. This is some leftover code from the old selection behavior that prevented visual selection of the left side of a diff if the user clicked on the right -- basically, a much simpler attack on what ultimately landed in D20191.

I think the change from `th` to `td` "broke" it so it didn't interfere with the other behavior, which is why I didn't have to remove it earlier. It's no longer necessary, in any case.

Test Plan: Grepped…