drive: Remove util::MigrateCacheFilesFromOldDirectories

This migration code has been shipped for 3 milestones.

BUG=None
TEST=unit_tests

Review URL: https://codereview.chromium.org/27663002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229296 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229296