nixos/programs/bash: Let bash-completion lazy load scripts

As described in detail here: https://github.com/NixOS/nixpkgs/issues/32533
bash will load completion scripts in $p/share/bash-completion/completions/ on
startup instead of letting bash-completion do it's lazy loading. Bash startup
will then slow down (very noticeable when bash-completion is installed in a
profile).

This commit leaves loading of scripts in the hands of bash-completion,
improving startup time for everyone using `enableCompletion`.

fixes #32533