Mark metrics obsolete due to AEC2 deprecation.

The acoustic echo canceler AEC2 is being deprecated. There are already no clients reporting *AecDelayAdjustmentMsSystemValue. *AecDelayAdjustmentMsAgnosticValue reports have drastically decreased since the AEC3 rollout, and are practically non-existent in M73+.

Bug: webrtc:10563
Change-Id: I8999169ac4f236296cd1873289c423745edb57d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572349
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Sam Zackrisson <saza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653503}