Removed dead "io.BytesIO vs cStringIO.StringIO" code

io.BytesIO is available since Python 2.6; Tornado doesn't support Python 2.5, so there won't be ImportError.

There is similar code in maint folder; I haven't changed it.

If the intention was to use (persumably) faster cStringIO in Python 2.x then instead of removing the conditional imports they should be changed to 

```
try:
    from cStringIO import StringIO as BytesIO # py2
except ImportError:
    from io import BytesIO # py3
```