Remove python-2.6 StringIO.StringIO fallback.

Remove python-2.6 StringIO.StringIO fallback.

Since the io module in python-2.6 was broken when threading was
disabled, we needed to fall back from io.StringIO to StringIO.StringIO
in this case (typically just for Gentoo's stage1 and stage2 tarballs).
Now that python-2.7 is stable in stages and we rely on io.open() being
available, we can also rely on io.StringIO being available.