Remove base-sandbox, which nobody uses.

BUG=chromium:807158
TEST=Built locally.

Change-Id: I22f1cd84fa10f003038bb5e42d77005715c5add5
Reviewed-on: https://chromium-review.googlesource.com/903207
Commit-Ready: Hidehiko Abe <hidehiko@chromium.org>
Tested-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Eric Caruso <ejcaruso@chromium.org>