File API: Remove vestigial //content/renderer/blob_storage.

The last code deletion CL landed on 8/1, and it's safe to assume we
won't need to revert it. So, it's time to remove the last remaining file
(OWNERS) from the directory.

Bug: 611935
Change-Id: Ic1f56b3d41482a969f305aa0672b430cd688cfea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1747162
Auto-Submit: Victor Costan <pwnall@chromium.org>
Commit-Queue: Staphany Park <staphany@chromium.org>
Reviewed-by: Staphany Park <staphany@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685882}