[FIX] registry: avoid dropping indexes

When an index has name 'tablename_fieldname_index', this index is
dropped if the field has index=False.  This lead to dropping a
user-created index or dropping/recreating the index when index=True is
set in a dependent module.  Some info is logged instead.

closes odoo/odoo#66700

closes odoo/odoo#71761

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>