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 invalidate utf8 sequences with this hack.

Test Plan: grepped for calls to this method

Reviewers: jungejason, aran, tuomaspelkonen

CC:

Differential Revision: 515