MAINT: spatial: static, unused code, sqrt(sqeuclidean)

I'm working on a big refactor of the `spatial.distance` C code, to get rid of all the repetitive code. This PR holds some preliminary work: removal of a lot of unused code, unnecessary inclusion of headers, pollution of the global linker namespace, a repetitive Euclidean distance kernel.

This PR is ready for review & merge. The bigger changes that I'm planning will be more controversial, so I'll open a separate PR for those.