Revert "enable bash autocomplete by default"

This reverts commit ee8e15fe76a235ae3583d4e8cb4bb370f28c5eae. See
discussion at https://github.com/NixOS/nixpkgs/commit/ee8e15fe76a235ae3583d4e8cb4bb370f28c5eae.

nixos/modules/programs/bash/bash.nix

···
       };
 
       enableCompletion = mkOption {
-        default = true;
+        default = false;
         description = ''
           Enable Bash completion for all interactive bash shells.
         '';