[FIX] account: cumulated balance non exportable in GL

We made the field cumulated balance non exportable
in the GL.

Steps:

- Go to Accounting->Accounting->General Ledger
- Unfold and select one or several lines
- export lines
-> The cumulated balance is not computed

The reason is we don't pass in the compute as we don't
come from the search_read method when exporting, so we don't have a domain
to compute the cumulated balance.
As we can't force the domain, we override the fields_get method
to make the field non exportable.

opw-2800669

closes odoo/odoo#91527

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