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-Commit-Position: refs/heads/master@{#19729}
diff --git a/webrtc/logging/rtc_event_log/rtc_event_log.h b/webrtc/logging/rtc_event_log/rtc_event_log.h
index 087f5d9..1e4650c 100644
--- a/webrtc/logging/rtc_event_log/rtc_event_log.h
+++ b/webrtc/logging/rtc_event_log/rtc_event_log.h
@@ -55,7 +55,6 @@
 }  // namespace rtclog
 
 class Clock;
-class RtcEventLogImpl;
 struct AudioEncoderRuntimeConfig;
 
 enum class MediaType;