Delete delta_sigma from repo

[Andrew Hearin]

As pointed out by @Christopher-Bradshaw, there should really not be more than one way to compute the same quantity. Even though the mean_delta_sigma function is ~30-50% slower than delta_sigma, this is outweighed by the improved accuracy. In any case, this performance hit will be rectified in the v1.0 release when all the functions in the mock_observables sub-package are replaced with versions that use mpi4py to parallelize across GPU-accelerated nodes, so the slow performance of mock_observables is a temporary problem anyway. So the old delta_sigma function should be deleted outright, along with all references to it in the docs and tutorials.

[Andrew Hearin]

This was done in #980.