Remove opacity effects for left-side / right-side diff text selection

Summary:
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.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20264