sage: remove duplicated pkgconfig

[SuperSandro2000]

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[collares]

This broke sage (build logs, but note that this also failed on ofBorg before you committed it). The pkgconfig you removed lives at pkgs/development/python-modules/pkgconfig, it's not the alias to pkg-config. Please tag the sage maintainers on sage changes.

cc @omasanori @timokau @7c6f434c

[collares]

By the way, the commit that added the "duplicated" pkg-config line explicitly says that sage breaks without it: d2c4b57