BackgroundSync: Remove "BackgroundSync" flag from RuntimeEnabledFeatures

BackgroundSync was shipped in milestone 49 and the flag is no longer necessary:
https://www.chromestatus.com/feature/6170807885627392

Bug: 449443
Change-Id: I77570e5d66f46d504fa6511ddefaf2d9790bc8ad
Reviewed-on: https://chromium-review.googlesource.com/603193
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492311}
6 files changed