.github/PULL_REQUEST_TEMPLATE: remove sandbox checkbox

This checkbox has been used very incosistently and its meaning is not
entirely clear: What does it mean if both checkboxes are *unchecked*?
Does that mean the sandbox was disabled? Or does it mean the checkbox
was just not handled?

Also the new nixpkgs-review-gha, which is increasingly used to test
builds on darwin platforms shows this information as part of the review
- where it's in a much better place.

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,9 +15,6 @@ For new packages please briefly describe the package or provide a link to its ho
   - [ ] aarch64-linux
   - [ ] x86_64-darwin
   - [ ] aarch64-darwin
-- For non-Linux: Is sandboxing enabled in `nix.conf`? (See [Nix manual](https://nixos.org/manual/nix/stable/command-ref/conf-file.html))
-  - [ ] `sandbox = relaxed`
-  - [ ] `sandbox = true`
 - [ ] Tested, as applicable:
   - [NixOS test(s)](https://nixos.org/manual/nixos/unstable/index.html#sec-nixos-tests) (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
   - and/or [package tests](https://github.com/NixOS/nixpkgs/blob/master/pkgs/README.md#package-tests)