[tracing] Cleanup up trace_log

This CL cleans up some of the unused functionalities in trace_log
- legacy metadata
- member AddTraceEvent* function: replaced by static ones
- SetTrackDescriptor: covered by TrackNameRecorder
    We no longer update SetTrackDescriptor on UpdateProcessLabel;
    this is unnecessary because TrackNameRecorder calls
    SetTrackDescriptor when the session ends.

Bug: 343404899
Change-Id: Ia3c4524cd3b3dd4d4ad12a0348e60480b5d03f67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6120057
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Mikhail Khokhlov <khokhlov@google.com>
Cr-Commit-Position: refs/heads/main@{#1403710}
NOKEYCHECK=True
GitOrigin-RevId: 235af4663543443c2af5913df46b8ba984b0e078