GTTF: Remove obsolete sharding_supervisor

This is a relanding of https://codereview.chromium.org/65143003/

BUG=236893

Review URL: https://codereview.chromium.org/66213009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234144 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234144