Remove RtcEventLogImpl forward declaration from rtc_event_log.h

The forward delcaration is not necessary.

BUG=None

Review-Url: https://codereview.webrtc.org/3006213002
Cr-Original-Commit-Position: refs/heads/master@{#19729}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5a7dad8453956007d83187cd8a591faf46c0b6c1
diff --git a/logging/rtc_event_log/rtc_event_log.h b/logging/rtc_event_log/rtc_event_log.h
index 087f5d9..1e4650c 100644
--- a/logging/rtc_event_log/rtc_event_log.h
+++ b/logging/rtc_event_log/rtc_event_log.h
@@ -55,7 +55,6 @@
 }  // namespace rtclog
 
 class Clock;
-class RtcEventLogImpl;
 struct AudioEncoderRuntimeConfig;
 
 enum class MediaType;