DEP : removed levypdf from mlab

This code raised exceptions and had the incorrect formula.  Given that
#3026 is the first we heard about this, it is unlikely anyone is
actually using so it can be safely removed.

As an alternative `scipy.stats.levy` should be used.

replaces #3026 and #3027

Going to hold off on getting rid of `normpdf` and `entorpy` for a more through check of what in mlab should stay.  My understanding is that many of those functions are there from before numpy/scipy matured and there are now better versions in those packages.