[FIX] base: prevent selecting unsupported "action types" in scheduled actions

Currently, scheduled Actions only work if you choose "Python Code" as
the action type ("Action To Do"). Other action types are not really
supported as they work "record-wise" (using active_ids) which makes no
sense for scheduled actions.

We should help users avoid surprises by hiding the "Action To Do" field
on scheduled actions, and default to "Python Code" implicitly.

Example steps to reproduce :

- go to scheduled actions > create a new action
- select "Execute several actions" as action to do
- add several actions to execute
- run this action

=> the sub-actions were not executed

opw-2227082

closes odoo/odoo#51215

X-original-commit: 06c72417
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>