PNaCl: Remove friendly reminder about new manifest format for optlevel.

It's been around for ~8 weeks. Assume there aren't too many
developers who haven't seen it yet.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3553
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/24195026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225516 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225516