[16.0][FIX] server_action_mass_edit: Don't do onchange

[pedrobaeza]

This change was introduced unnoticed in the migration to 16.0 in #544, but it has several problems:

• The onchange is only executed for one record, not several, and this module is for mass actions.
• Not all the users want to play onchanges, being for one record or several.

Let's remove it for being consistent. An ongoing discussion for supporting onchanges through onchange_helper is meanwhile active.

@Tecnativa