Remove unused method PrefService::ReloadPersistentPrefs().

This was removed as part of removing the import process in issue 219419.

BUG=219419
R=battre@chromium.org

Review URL: https://codereview.chromium.org/24453002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225185 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225185