Use a map instead of a hook to jump to grep result

[lobre]

This has been discussed a little bit on Discord, but I think we do not need a hook for the <ret> mapping in the grep module.

I guess this was something that was necessary in the past, but I am not sure that we need it anymore.

Also, it prevents from overriding the <ret> key easily at the user level with:

map global normal <ret> "..."