ceph: remove unused dependency from httpcore

This fixes the build of ceph and can be reverted once
https://github.com/NixOS/nixpkgs/pull/462435 reaches master.

pkgs/by-name/ce/ceph/package.nix

@@ -321,6 +321,9 @@ let
           };
         });
 
+        httpcore = super.httpcore.overridePythonAttrs (old: {
+          nativeCheckInputs = lib.remove self.pproxy old.nativeCheckInputs;
+        });
       };
   };