Before this commit, the field cash_basis_base_account_id, used
to keep track of the base on which a tax a computed for cash basis entries
was required in the tax form view

Since this account can be absent, and will be most of the time, and that
an account is retrieved anyway in the move creation process it is safe
to remove the required attribute in the view

After this commit, the field is not required in view anymore

OPW 2072750

closes odoo/odoo#37060

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>