The unused properties fields number is calculatable via used in-object
properties count and we can drop it now.

Bug: chromium:774644
Change-Id: I7388af7772a8e793593fabc46527886cf2e36095
Reviewed-on: https://chromium-review.googlesource.com/781465Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49542}