[17.0][FIX] hr_timesheet_sheet: don't block the change of period

[MiquelRForgeFlow]

When you are creating the sheet, it automatically creates sheet lines for the existing analytic lines. But if you change the sheet period before saving, don't delete those analytic lines.

How to trigger the error (easy way):

• Create a time-off for a day of this week.
• Approve the time-off.
• Start to create timesheet sheet (don't save it). Note that a sheet line has been create for that time-off.
• Change the sheet period to the previous week (first change the start date, then the end date). Note that the sheet line for the time-off is not visible anymore.
• Try to save the sheet, but a blocker is shown instead:

[sbejaoui]

We resolved this issue with a different approach in #848. The date fields are read-only after the save, which makes the onchange unnecessary.