chore(algebraic_geometry/Spec): reduce imports (#4007)
The main change is to remove some `example`s from `topology.category.TopCommRing`, so that we don't need to know about the real and complex numbers on the way to defining a `Scheme`.
While I was staring at `leanproject import-graph --to algebraic_geometry.Scheme`, I also removed a bunch of redundant or unused imports elsewhere.
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
