[Extensions Metrics] Stop recording Extensions.ExtensionCreationTime.*

Stop recording Extensions.ExtensionCreationTime.* and mark it as
obsolete in histograms.xml. We haven't been looking at it lately, so we
shouldn't continue recording it. We can always add it back later if we
need it again.

Bug: 917738
Change-Id: If3711f0e04de29fb5f4e00acdada2b97fbd37d8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600854
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658253}