Cleanup stale preference_hashes on platforms where pref hash support was temporarily removed.

BUG=343261

Review URL: https://codereview.chromium.org/185683006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255037 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255037