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