Remove C++11 calls from intelligibility_utils

The C++11 here was overkill. This replaces it with simpler logic that
covers all cases encountered so far in practice.

The problem was previously brought up here: https://codereview.webrtc.org/1250663007/

BUG=427718, 487341, webrtc:4866
R=andrew@webrtc.org

Review URL: https://codereview.webrtc.org/1242943008

Cr-Commit-Position: refs/heads/master@{#9627}
3 files changed