[FIX] l10n_es_edi_sii: the 21% ISP tax in itself is not isp

The tax 21% ISP actually behaves like a normal 21% tax
(it gets reported in the same lines, ...)

But the reason for its existence is that in case of the
fiscal position Revendedor ISP...  it gets mapped to the
0% ISP tax (which needs to be 0)

Before, if you would however send a 21% ISP tax to the SII,
it would be set as ISP and you would get an error telling
that the tax rate should be 0 in case of ISP.  (but now, it
will be reported as S1 and not trigger the error)

See also discussion in
https://github.com/odoo/odoo/pull/119339#issuecomment-1519720558



closes odoo/odoo#119603

Signed-off-by: Olivier Colson (oco) <oco@odoo.com>