[FIX] server_environment: compute_default assign

Revert "server_environment: Fix value assignation in compute from default"

This reverts commit cc56952.

The fix is breaking other modules, the original problem must be fixed in
mail_environment.

server_environment/models/server_env_mixin.py

@@ -254,7 +254,7 @@ def _compute_server_env_from_config(self, field_name, options):
 
     def _compute_server_env_from_default(self, field_name, options):
         if options and options.get("compute_default"):
-            self[field_name] = getattr(self, options["compute_default"])()
+            getattr(self, options["compute_default"])()
         else:
             default_field = self._server_env_default_fieldname(field_name)
             if default_field: