move o.systemConfiguration to initramfs module

as far as I can tell, we define it identically in every module
that uses initramfs

modules/outputs/ext4fs.nix

@@ -20,8 +20,6 @@ in
     };
     boot.initramfs.enable = true;
     system.outputs = {
-      systemConfiguration =
-        pkgs.systemconfig config.filesystem.contents;
       rootfs =
         let
           inherit (pkgs.pkgsBuildBuild) runCommand e2fsprogs;