Remove UTF-8 kludges from Differential

Summary:
This depends on D513 and D514. Those diffs make the display algorithms safe with
respect to mutating utf8, so we no longer need to repair potentially invalid
utf8 sequences with this hack.

Test Plan:
grepped for calls to this method

Reviewers: jungejason, aran, tuomaspelkonen
Commenters: jungejason
CC: aran, epriestley, jungejason
Differential Revision: 515