import: Remove Nix importer.

import: Remove Nix importer.

This importer has suffered from bitrot and no longer works with current
Nix and Nixpkgs.  See <https://bugs.gnu.org/32339> and
<https://bugs.gnu.org/36255>.

* guix/import/snix.scm, guix/scripts/import/nix.scm,
tests/snix.scm: Remove.
* Makefile.am (MODULES, SCM_TESTS): Remove them.
* guix/scripts/import.scm (importers): Remove "nix".
* build-aux/test-env.in: Remove NIXPKGS variable.
* configure.ac: Remove '--with-nixpkgs' option.
* doc/guix.texi (Invoking guix import): Remove bit about "guix import
nix".
* etc/completion/fish/guix.fish: Likewise.