[IMP] *: remove `digits` attribute from Monetary fields

* account, hr_contract, hr_expense, point_of_sale, sale_margin,
website_sale_delivery, website_sale_wishlist

digits only works on Float fields, on Monetary fields it has no
effect:

- The column_type of a monetary field is always numeric
- It is not applied in `convert_to_column()` or `convert_to_cache()`
- It is not part of `description_attrs`, so it will not be included in
the output of `fields_get()`

closes odoo/odoo#35336

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>