Remove documentation for old MLContext API

[fschueler]

As a step to deprecate the old MLContext API, I suggest to remove its 
documentation for the next release (together with a deprecation of the 
actual API so that we can remove it in 1.0).

Currently the section about the old API is placed in between up-to-date 
documentation and makes it pretty confusing to see what is old and what 
is new.

Any objections? Alternatively we could put it all the way to the end or 
in a separate document.

-Felix