Remove unused files json_response_fetcher.{cc,h}

The class JSONResponseFetcher is never instantiated and the header
never included (apart from the implementation file) so as far as I
can tell this is dead code and safe to remove.

Bug: none
Change-Id: Ib9d1c11f744f90111d85172c00dacc7ba8e74d3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611598
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659866}
3 files changed