nixos/environment: stop setting ASPELL_CONF

We set[1] ASPELL_CONF to the last nix profile containing lib/aspell in
2013.  In 2017, aspell is patched[2] to search NIX_PROFILES, which
makes [1] not needed any more.

Deleting it is also agreed in this discussion[3].

[1]: 0192c02720
[2]: ba4cefe4ae
[3]: https://github.com/NixOS/nixpkgs/pull/30234

nixos/modules/programs/environment.nix

@@ -51,13 +51,6 @@ in
 
     environment.extraInit =
       ''
-         unset ASPELL_CONF
-         for i in ${concatStringsSep " " (reverseList cfg.profiles)} ; do
-           if [ -d "$i/lib/aspell" ]; then
-             export ASPELL_CONF="dict-dir $i/lib/aspell"
-           fi
-         done
-
          export NIX_USER_PROFILE_DIR="/nix/var/nix/profiles/per-user/$USER"
          export NIX_PROFILES="${concatStringsSep " " (reverseList cfg.profiles)}"
       '';