drive: Remove unused FileCache::GetFileOnUIThread

BUG=231221
TEST=unit_tests

Review URL: https://chromiumcodereview.appspot.com/21215002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224951 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224951