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

Made the functions in distance_wrap.c static. No need to pollute the global
linker namespace with these; they're passed to Python using a table of
function pointers.
Merge pull request #3722 from larsmans/distance-cleanup

MAINT: spatial: extern -> static, remove unused code, sqrt(sqeuclidean)