filter for submenu_as_parent parameter

[plocha]

This filter enables plugins to remove the menu entries which have the same target as their parents.

We could use this filter to determine whether /wp-admin/menu.php and add_submenu_page should add 'same target' submenu entries. But I think there is no need for it because some plugins manually add such submenu entries. So plugins which use this filter should remove them manually.

[plocha]

Duplicate of #19085.

[markoheijnen]

Clearing out the milestones for closed tickets on Awaiting Review