Revert "enable bash autocomplete by default"

This reverts commit ee8e15fe76. See
discussion at ee8e15fe76.

nixos/modules/programs/bash/bash.nix

@@ -105,7 +105,7 @@ in
       };
 
       enableCompletion = mkOption {
-        default = true;
+        default = false;
         description = ''
           Enable Bash completion for all interactive bash shells.
         '';