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}
1 file changed