lumpy-sv: remove skip_clean :all

This has been a no-op since
Homebrew/homebrew@c87736e7b9ded6ccef805f807d5844a1e948c3cc.
mathgl: remove BuildOptions monkeypatch

Only the interface exposed via the `build` attribute is public. The
BuildOptions constant itself is an implementation detail.
Stop using class-level build.stable?, build.devel?, and build.head?

Using these methods in the class body will soon emit a deprecation
warning (only in the class body, they will continue to be supported in
instance methods).