Limit the menu level on node add/edit page
Menu Performance Module does exactly that. 

Drupal is already aware of this issue and has a variable menu_override_parent_selector which can be leveraged by other modules to come up with their own implementation of the menu handlers on node edit page.

As the project page reads 


  Menuperformance takes advantage of this, and sets the variable when
  installed. It then proceeds to add its own AJAX based parent menu item
  selector widgets for the node edit, node type edit and vocabulary edit
  pages.


How to Use from README.txt


  
  Enable the module.
  Go to /admin/config/user-interface/menuperformance , and activate the  module. Checking the checkbox sets Drupal variable 
  menu_override_parent_selector.
  After the module is activated, the menu item selectors on the above  mentioned pages should have been replaced with a multiple dropdown 
  AJAX-powered selector instead of the standard single dropdown.