Delete main_nav_parent and user_nav_parent system settings

[Ruslan-Aleev]

Feature request

Summary

What are these main_nav_parent and user_nav_parent system settings for?

The position of menu items can be changed in the "Menu" section, there are fixed areas (Main menu and User menu), and additional system settings only confuse.
For example, I set the user_nav_parent setting to topnav, expecting the user menu to appear in the main menu, but no, the menu just disappeared, and the main menu now appeared in the user menu.
Also the language selection menu breaks if the menu is placed on top.

Suggested solution(s)

Delete main_nav_parent and user_nav_parent system settings.
No confusion, less code and lexicons.

Related issue(s)/PR(s)

#14539