nixos: enable bash command completion by default

Because it improves out-of-the-box user experience a lot (IMHO).
(zsh completion is already on by default.)

Remove "programs.bash.enableCompletion = true" from
nixos-generate-config.pl, which feels superflous now.

nixos/modules/programs/bash/bash.nix

@@ -110,7 +110,7 @@ in
       };
 
       enableCompletion = mkOption {
-        default = false;
+        default = true;
         description = ''
           Enable Bash completion for all interactive bash shells.
         '';