flood: fix mainProgram merge issue

The package was repackaged using `buildNpmPackage` ([1]). In parallel
its `meta.mainProgram` was defined for the old `node-packages.nix`
packaging format ([2]).

Both branches were merged almost at the same time, breaking the build.

[1]: https://github.com/NixOS/nixpkgs/pull/245435
[2]: https://github.com/NixOS/nixpkgs/pull/247095

pkgs/applications/networking/p2p/flood/default.nix

···
     homepage = "https://flood.js.org";
     license = licenses.gpl3Only;
     maintainers = with maintainers; [ thiagokokada winter ];
+    mainProgram = "flood";
   };
 }

pkgs/development/node-packages/main-programs.nix

···
   firebase-tools = "firebase";
   fkill-cli = "fkill";
   fleek-cli = "fleek";
-  flood = "flood";
   git-run = "gr";
   gitmoji-cli = "gitmoji";
   graphql-cli = "graphql";