Remove mention of TODO file

The TODO file itself was removed here:

    commit 62378533298754a9cb9475813124c6c9f1c174b0
    Author: Sanko Robinson <sanko@cpan.org>
    Date:   Mon Jun 30 06:47:59 2014 -0400

        Deprecation of Readonly::XS as a requirement on modern perl is complete

        - Removing TODO
        - Major version bump to undo my screw up
Merge pull request #14 from pmorch/master

Remove mention of TODO file