[IMP] purchase: set schedule_date of po lines as per date_planned of PO

In this commit -
1. Previously, date_planned of PO was computed and set while the creation of PO.
   In this commit, removed the compute logic and now field will be empty until
   user fills it.
2. ‘Set date to all order lines’ button is removed.
3. Now if user have filled the date_planned of PO, it will be copied to all po
   lines.
4. Schedule date of PO line will be readonly if date_planned have any value
   else will be editable.
5. Update the test cases of computation of date_planned.

Task-2032417