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## Short functional explanation of the error
Let's say we create a role containing employees with fixed schedules. Then, we create a shift template that applies on this role. When we create a shift, the starting and ending hours will take into consideration the hours of the employee's fixed schedule, instead of aligning with the shift template start and end hours.

As discussed with XBO, the start and end hours of the shift should align with the shift template, despite the fixed working schedules having different start and end hours.

## Reproduction Steps
1. Go to Planning. Click on Configuration tab > roles.
2. Create a role and add an employee as a resource This employee has to have a fixed working schedule.
3. Click on Configuration tab > Shift Templates.
4. Create a new Shift Template. Select starting and ending hours different from the employee's fixed schedule. Select the role you just created.
5. Click on Schedule tab > By resource and click on New.
6. Select the role you just created.

### Expected behavior
The start and end hours should align with the shift template start and end hours.

### Unexpected behavior
The start and end hours are aligned on the employee's fixed working schedule: if the employee has a schedule from 8 to 16h36 and the shift template goes from 10 to 18, the starting and ending hours will be 10 to 16h36.

## Origin of the issue
We kept computing the working intervals of employees, even if a shift template was set:
https://github.com/odoo/enterprise/blob/8b00363e5e461f11b9736354d94e520e21932e71/planning/models/planning.py#L656-L664 Which isn't necessary in the case where a shift template has been set, as the start and end time are determined by the shift template, and not the employee's schedule.
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opw-5898509

Forward-Port-Of: odoo/enterprise#107018