Remove duplicated NoStepper class

There's also two duplicated functions in [distributions.py](https://github.com/pymc-devs/pymc/blob/master/pymc/distributions.py).
`noncentral_t_expval` and `noncentral_t_like` I think might be the result of unfinished copy & paste and must be renamed and adapted [here](https://github.com/pymc-devs/pymc/blob/master/pymc/distributions.py#L2504) and [here](https://github.com/pymc-devs/pymc/blob/master/pymc/distributions.py#L2513)?