CQ. Remove most of DeclarationsTracker.

Keep only enough to support DartdocDirectiveInfo.

Change-Id: I228d771b866bd7f0a903b62064f058c9edd04bee
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393020
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: scheglov