[IMP] account: faster account_move unlink

On huge DB, unlinking an account_move can take a lot of time.
Most of this time is wasted checking for foreign key constraint.

closes odoo/odoo#84817

Signed-off-by: William André (wan) <wan@odoo.com>

addons/account/models/account_move.py

@@ -297,8 +297,14 @@ class AccountMove(models.Model):
         help='If this checkbox is ticked, this entry will be automatically posted at its date.')
 
     # ==== Reverse feature fields ====
-    reversed_entry_id = fields.Many2one('account.move', string="Reversal of", readonly=True, copy=False,
-        check_company=True)
+    reversed_entry_id = fields.Many2one(
+        comodel_name='account.move',
+        string="Reversal of",
+        index='btree_not_null',
+        readonly=True,
+        copy=False,
+        check_company=True
+    )
     reversal_move_id = fields.One2many('account.move', 'reversed_entry_id')
 
     # =========================================================