Delete shell.nix (#3726)

It's broken. And, the official way to get a nix env for this project is
to use direnv.

Author: supersven

changelog.d/5-internal/delete-shell_nix

@@ -0,0 +1 @@
+Delete `shell.nix`. It has been broken for quite some time. The supported way to get a development nix environment is to use direnv.