Remove background_fetch directory in Blink public platform

This directory doesn't have any code. Remove it.

Bug: 919392
Change-Id: I69443b2d892b459dad3cc9a47b7a8445f28ac7c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412061Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#807425}

third_party/blink/public/platform/modules/background_fetch/OWNERS

-peter@chromium.org
-
-per-file *.mojom=set noparent
-per-file *.mojom=file://ipc/SECURITY_OWNERS
-
-# TEAM: platform-capabilities@chromium.org
-# COMPONENT: Blink>BackgroundFetch