Revert "Revert "Remove which -> type -P alias.""

This reverts commit ddd480ac30579d780c8ffa9c590a8c86bb36d8d2. Gave it
some more thought.

nixos/modules/profiles/base.nix

···
     pkgs.ddrescue
     pkgs.ccrypt
     pkgs.cryptsetup # needed for dm-crypt volumes
+    pkgs.which # 88K size
 
     # Some networking tools.
     pkgs.fuse

nixos/modules/programs/bash/bash.nix

···
       */
 
       shellAliases = mkOption {
-        default = config.environment.shellAliases // { which = "type -P"; };
+        default = config.environment.shellAliases;
         description = ''
           Set of aliases for bash shell. See <option>environment.shellAliases</option>
           for an option format description.