[FIX] account: Prevent exchange diff entry with full refund

- create an invoice with a CABA tax
- full refund it using 'add credit note', 'cancel' mode.
=> No CABA entry generated but an exchange difference journal entry
has been created.

This is because the CABA entry is not generated in case of full refund
thanks to the 'move_reverse_cancel' context key. However, this key is not
preventing the generation of the exchange journal entry for CABA.

opw-3321897

closes odoo/odoo#121597

Signed-off-by: Brice Bartoletti (bib) <bib@odoo.com>