Remove Roots_Nav_Walker as default for all menu

I love how `Roots_Nav_Walker()` create menu html.
But, I believe that apply by default to all menus is too much.

Often I use the menu widget to show complex menus with sub-element.
In these situations sub-elements are wrapped in dropdown menus, which is fine for primary menu but not for aside or footer.

I suggest to:
* remove `Roots_Nav_Walker()` as default for all menus
* add `Roots_Nav_Walker()` to Primary menu

In this way:
* if user wants another menu using `Roots_Nav_Walker()`, can simply copy and paste the code in `header.php`
* user can add other custom Nav_Walkers 
* Widget menus can have as many level as user likes.