treewide: drop copumpkin from maintainers

Their last commit is from March 28th 2019, about 6 years ago.

This PR is in no way intended to diminish Daniels's accomplishments, and they're welcome to just say so if they'd prefer this PR not to be merged. Also, even if it's merged, of course they're always welcome to return to activity and be added back. The intent of this PR is to give more realistic expectations around the maintainership of these packages, and to invite others to step up for maintainership if they rely on those packages.

If this is merged, they should probably also be removed from the list
of committers for the time being.

nixos/lib/make-disk-image.nix

@@ -447,8 +447,7 @@ let
     mkdir -p $root
 
     # Copy arbitrary other files into the image
-    # Semi-shamelessly copied from make-etc.sh. I (@copumpkin) shall factor this stuff out as part of
-    # https://github.com/NixOS/nixpkgs/issues/23052.
+    # Semi-shamelessly copied from make-etc.sh.
     set -f
     sources_=(${lib.concatStringsSep " " sources})
     targets_=(${lib.concatStringsSep " " targets})