desktop-sharing: remove obsolete metrics

This change:
* Removes metrics declarations for Sharing.SharingHubDesktop.TimeToFetch*,
  which were added as part of an experiment with an alternate design for
  the desktop sharing hub that was never shipped;
* Removes the metrics declaration and code to gather
  Sharing.SharingHubDesktop.TimeToShow, since nobody looks at this
  metric. This simplifies the code somewhat.

Fixed: 1495069
Change-Id: I5df3c144b2993c96cd84b0c00bf883b9bae85735
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4972736
Auto-Submit: Elly FJ <ellyjones@chromium.org>
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Commit-Queue: Jeffrey Cohen <jeffreycohen@chromium.org>
Reviewed-by: Jeffrey Cohen <jeffreycohen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1214367}