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/view/facility_form.xml

@@ -10,10 +10,6 @@
         <field name="commitment_status"/>
         <label name="currency"/>
         <field name="currency"/>
-        <label name="fx_feeder"/>
-        <field name="fx_feeder"/>
-        <label name="fx_haircut_pct"/>
-        <field name="fx_haircut_pct"/>
         <newline/>
         <label name="date_from"/>
         <field name="date_from"/>

modules/trade_finance/view/facility_limit_form.xml

@@ -20,7 +20,7 @@
         <field name="sequence"/>
         <newline/>
         <label name="parent"/>
-        <field name="parent" colspan="3"/>
+        <field name="parent" colspan="3" readonly="1"/>
         <label name="facility"/>
         <field name="facility" colspan="3"/>
         <label name="notes"/>