Remove unused QualityMetricsReporter::Start method from pc test api

Bug: webrtc:11381
Change-Id: I254aef4f0c218fe1ee70499f405449f8a226b44b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178567
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Andrey Logvin <landrey@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31618}
diff --git a/api/test/peerconnection_quality_test_fixture.h b/api/test/peerconnection_quality_test_fixture.h
index 94a2ad6..6dc119f 100644
--- a/api/test/peerconnection_quality_test_fixture.h
+++ b/api/test/peerconnection_quality_test_fixture.h
@@ -420,11 +420,6 @@
     // StopAndReportResults().
     virtual void Start(absl::string_view test_case_name,
                        const TrackIdStreamInfoMap* reporter_helper) = 0;
-    // This method has been added for backwards compatibility with upstream
-    // project.
-    void Start(absl::string_view test_case_name) {
-      Start(test_case_name, nullptr);
-    }
 
     // Invoked by framework after call is ended and peer connection factory and
     // peer connection are destroyed.