Remove EllipticalSlice for V4

[ricardoV94]

This step method has not been refactored yet:

pymc/pymc3/step_methods/elliptical_slice.py

Line 103 in 3cc457b

# XXX: This needs to be refactored

Since this is something we are simply not carrying over to v4, I don't think we need to deprecate it (since it won't work and we aren't going to port it). So this will be a removal, rather than a deprecation.