revert: Benchmark iteration warnings should warn

This commit reverts:

    commit a820780c62918236069b3b55502640c99a9e812a
    Author:     David Golden <dagolden@cpan.org>
    AuthorDate: Tue Feb 5 22:23:37 2013 -0500
    Commit:     David Golden <dagolden@cpan.org>
    CommitDate: Tue Feb 5 22:49:36 2013 -0500

        Benchmark iteration warnings should warn, not print

because the warnings appearing on STDERR weren't being trapped by
Benchmark.t (see [perl #116831]). Revert for now, and re-address after
5.18.