Remove tautological 'unsigned expr < 0' comparisons

This is the result of compiling Chromium with
Wtautological-unsigned-zero-compare. For more details, see:
https://chromium-review.googlesource.com/c/chromium/src/+/2802412

Change-Id: I05cec6ae5738036a56beadeaa1dde5189edf0137
Bug: chromium:1195670
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/213783
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33689}

api/audio_codecs/opus/audio_encoder_opus_config.cc

@@ -61,7 +61,7 @@ bool AudioEncoderOpusConfig::IsOk() const {
     // well; we can add support for them when needed.)
     return false;
   }
-  if (num_channels < 0 || num_channels >= 255) {
+  if (num_channels >= 255) {
     return false;
   }
   if (!bitrate_bps)