python3.pkgs.buildsetupcfg: Remove function from python-packages-base.nix

This function is not, and never have been, used anywhere inside nixpkgs, outside of bootstrapping setupcfg2nix itself.
It was added in https://github.com/NixOS/nixpkgs/pull/38778 by @shlevy.

It has no out-of-tree users on Github either. External breakage is not expected.

pkgs/development/interpreters/python/python-packages-base.nix

@@ -47,9 +47,6 @@ let
     toPythonModule = x: x;  # Application does not provide modules.
   }));
 
-  # See build-setupcfg/default.nix for documentation.
-  buildSetupcfg = import ../../../build-support/build-setupcfg lib self;
-
   # Check whether a derivation provides a Python module.
   hasPythonModule = drv: drv?pythonModule && drv.pythonModule == python;
 
@@ -92,13 +89,11 @@ let
   disabledIf = x: drv: if x then disabled drv else drv;
 
 in {
-
   inherit lib pkgs stdenv;
   inherit (python.passthru) isPy27 isPy37 isPy38 isPy39 isPy310 isPy311 isPy3k isPyPy pythonAtLeast pythonOlder;
   inherit buildPythonPackage buildPythonApplication;
   inherit hasPythonModule requiredPythonModules makePythonPath disabled disabledIf;
   inherit toPythonModule toPythonApplication;
-  inherit buildSetupcfg;
 
   python = toPythonModule python;
   # Dont take pythonPackages from "global" pkgs scope to avoid mixing python versions