[WIP] remove check_arrays stuff and old input validation

Follow-up on #3443, removing all the old stuff.
On 07/20/2014 01:25 PM, Olivier Grisel wrote:
>
> In doc/developers/utilities.rst:
>
> >
> > -- :func:`check_arrays`: check that all input arrays have consistent first
> > -  dimensions.  This will work for an arbitrary number of arrays.
> > +- :func:`sliceable`: check that all input arrays have consistent length and can
>
> What about renaming this to |indexable|? Or even |check_indexable|?
>
I renamed it to indexable.