Remove opacity effects for left-side / right-side diff text selection
These effects feel like they're possibly overkill, since other CSS rules make the selection reticle behave correctly and the implementation is relatively intuitive.
Or not, either way.
Test Plan: Selected text on either side of a 2-up diff, no more opacity effects.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20264