[FIX] ir_model: properly delete m2m whose comodel is being deleted

Before this patch, if an m2m's relation field pointed to a model that
was to be deleted, the deletion of the aforementioned model wouldn't
trigger the m2m's deletion, which would lead to the registry crashing.

With this patch, whenever an ir.model is being unlinked, a check is
performed to verify that no other fields (m2m or otherwise) have this
model as a comodel, and if they do, they are unlinked as well.

closes odoo/odoo#30230

odoo/addons/base/ir/ir_model.py

@@ -180,6 +180,9 @@ class IrModel(models.Model):
                 # prevent screwing up fields that depend on these models' fields
                 model.field_id._prepare_update()
 
+        # delete fields whose comodel is being removed
+        self.env['ir.model.fields'].search([('relation', 'in', self.mapped('model'))]).unlink()
+
         self._drop_table()
         res = super(IrModel, self).unlink()