Remove unused class from sage_setup.optional extension #31013
Description
The code in optional_extension was mostly unused and is removed in this ticket (and similarly extension.skip_build was nowhere else set).
The only still used method was is_package_installed_and_updated which is moved to sage.misc.package.
Related: #28815
Component: build
Keywords: sd111
Author: Tobias Diez
Branch/Commit: a7b1e81
Reviewer: François Bissey
Issue created by migration from https://trac.sagemath.org/ticket/31013