[FIX] web: remove prohibited dropzones of the menu editor

In the website menu editor and in the studio menu editor, the user can
drag & drop the elements that constitute the menu of his website/app.
Users can also put a menu into another menu to create a sub-menu. For
the website, we allow two levels of menu but not more. For studio we
allow 5 levels of menu. When the user starts to drag an item, dropzones
can be drawn on the prohibited level (3 in website, 6 in studio) while
he can't create this level of menu. This commit adds a css rule to hide
those forbidden dropzones.

task-3251032

closes odoo/odoo#117300

Signed-off-by: loco-odoo <loco@odoo.com>