[REF] stock: remove index on date of stock.picking

The `date` fields is only used in the `_accumulate_amounts`  of
point_of_sale and is set at now or by the `_prepare_picking` of purchase
(maybe will be remove in the futur).
In all case, and for now, the index is not
used (expected by at a custom search) and should be remove.

task-2389380

addons/stock/models/stock_picking.py

@@ -294,7 +294,7 @@ class Picking(models.Model):
         help="Is late or will be late depending on the deadline and scheduled date")
     date = fields.Datetime(
         'Creation Date',
-        default=fields.Datetime.now, index=True, tracking=True,
+        default=fields.Datetime.now, tracking=True,
         states={'done': [('readonly', True)], 'cancel': [('readonly', True)]},
         help="Creation Date, usually the time of the order")
     date_done = fields.Datetime('Date of Transfer', copy=False, readonly=True, help="Date at which the transfer has been processed or cancelled.")