Remove old scipy cruft

[jarrodmillman]

@stefanv This needs more work. In skimage/segmentation/tests/test_random_walker.py, we have

# older versions of scipy raise a warning with new NumPy because they use
# numpy.rank() instead of arr.ndim or numpy.linalg.matrix_rank.
SCIPY_RANK_WARNING = r'numpy.linalg.matrix_rank|\A\Z'
PYAMG_MISSING_WARNING = r'pyamg|\A\Z'
PYAMG_OR_SCIPY_WARNING = SCIPY_RANK_WARNING + '|' + PYAMG_MISSING_WARNING
NUMPY_MATRIX_WARNING = None

I don't think we need SCIPY_RANK_WARNING, PYAMG_OR_SCIPY_WARNING, or NUMPY_MATRIX_WARNING now. Thoughts?

[stefanv]

Yes, looks like we can only use PYAMG_MISSING_WARNING.

[lagru]

Looks good. Thanks Jarrod.