systemd: drop -Defi-ld=gold

As reported in
https://github.com/NixOS/nixpkgs/pull/156096#pullrequestreview-900986176,
this fails to build on EFI enabled RISC-V because the requested EFI
linker (efi-ld=gold) is unsupported. According to Wikipedia gold only
supports x86, x86-64, ARM, PowerPC, TileGX.

Removing this option alltogether will cause meson to figure out the
default linker by itself.

pkgs/os-specific/linux/systemd/default.nix

@@ -460,7 +460,6 @@ stdenv.mkDerivation {
     "-Dsmack=true"
     "-Db_pie=true"
     "-Dinstall-sysconfdir=false"
-    "-Defi-ld=gold"
     "-Dsbat-distro=nixos"
     "-Dsbat-distro-summary=NixOS"
     "-Dsbat-distro-url=https://nixos.org/"