Remove fx fields from Facility; fix facility_limit tree and parent field

- Remove fx_feeder and fx_haircut_pct fields from Facility model and form view
- Add field_childs to facility_limit tree view for hierarchical display
- Set parent field as readonly in facility_limit form

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/trade_finance/facility.py

@@ -91,10 +91,6 @@ class Facility(ModelSQL, ModelView):
 
     currency = fields.Many2One('currency.currency', 'Facility Currency',
         required=True, ondelete='RESTRICT')
-    fx_feeder = fields.Many2One('trade_finance.fx_feeder', 'FX Rate Feeder',
-        ondelete='RESTRICT')
-    fx_haircut_pct = fields.Numeric('FX Haircut (%)', digits=(16, 2))
-
     is_tpa = fields.Boolean('TPA',
         help='Third Party Account — broker managed structure')
     broker = fields.Many2One('party.party', 'Broker',