Don't monkeypatch doctest during IPython startup.

This is no longer required since Python [issue 8048](http://bugs.python.org/issue8048) was fixed in 2.6.6, 2.7.1 and 3.2.

It should buy us another small improvement in startup time, because we no longer need to load doctest on startup. That saves 29 modules on 2.7, and 8 on 3.2. Cold start time appears to drop by something like 10%, although there's quite a bit of noise in the measurements.

I've left the module `IPython.utils.doctestreload` around for now, in case anyone is working with earlier versions of Python and needs to use it.