Stop collecting data for StreamBrokerDisconnectReason

This histogram has been tagged obsolete for 9 months,  let's remove it.

Bug: 974918
Change-Id: Ic6a260147a80fc29c737bd127517827bcf69344f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663503
Auto-Submit: Jonas Olsson <jonasolsson@chromium.org>
Commit-Queue: Olga Sharonova <olka@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672265}

content/browser/media/audio_input_stream_broker.cc

@@ -131,9 +131,6 @@ AudioInputStreamBroker::~AudioInputStreamBroker() {
   TRACE_EVENT_NESTABLE_ASYNC_END1("audio", "AudioInputStreamBroker", this,
                                   "disconnect reason",
                                   static_cast<uint32_t>(disconnect_reason_));
-
-  UMA_HISTOGRAM_ENUMERATION("Media.Audio.Capture.StreamBrokerDisconnectReason",
-                            disconnect_reason_);
 }
 
 void AudioInputStreamBroker::CreateStream(