[FIX] base: allow deletion of inherited custom field

The inherited field of a custom field cannot be deleted

Steps to reproduce:
1. Install Contacts and Studio
2. Go to Contacts and open any contact
3. Toggle Studio
4. Add a field of any type in the view, remove it and close Studio
5. Go to Settings > Technical > Database Structure > Fields and search
   for `x_studio`

Solution:
Mark the inherited field as manual if its parent field is manual and
allow the deletion of inherited custom field if we also delete its
dependency

Problem:
The inherited field was not marked as custom so it was impossible to
delete it

opw-3093581

closes odoo/odoo#130420

Signed-off-by: Raphael Collet <rco@odoo.com>