[FIX] SSF: misinterpretation of onchange command (4)

From an onchange, at this point, (4) doesn't mean "no change" it means
"reset to database values". Through the interplay of client and server
reverse-engineering one another at this point the client (is supposed
to) assume the o2m results are "complete" and a diff from the
current *in-database* values rather than the in-client (sent to the
server) ones.

So a (1) should completely replace all existing values, and a (4)
should just remove all of them (but keep the record linked).

closes odoo/odoo#32617

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>