Remove rtc::ConstantLabels from webrtc_overrides.

Constant Labels was only used by //third_party/libjingle_xmpp.

In CL [1] I removed this error handling mechanism from libjingle_xmpp
so it is possible to remove this code and simplify
//third_party/webrtc_overrides.

[1] - https://chromium-review.googlesource.com/c/chromium/src/+/1397607

Bug: 896154
Change-Id: I3b4d8869d7813c789b7b12f8cad0fdc079796491
Reviewed-on: https://chromium-review.googlesource.com/c/1397982
Reviewed-by: Tommi <tommi@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625143}
2 files changed