Remove unused VariationsSeedExpiry enum.

It now lives in variations_field_trial_creator.cc, so this copy
is not used.

Bug: none
Change-Id: I8bccc220e29b550a5b45ebd95aa557f3cf2ac64b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2305112


Auto-Submit: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791459}