Remove stale thumbnail URL checker.

We have already switched to the permanent thumbnail URLs (r226362).
Let's remove the caching and refreshing of them.

BUG=301569

Review URL: https://codereview.chromium.org/31643002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230026 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230026