Motivation for this change

From the release notes: After upgrade to 0.15.0, some clients would crash at startup because a custom fee setting was configured that no longer exists in the GUI. This is a minimal patch to avoid this issue from occuring.

I have also removed the sourceforge mirror as bitcoin sources have not been hosted on sourceforge for a number of years now.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.