[REF] resource_work_time_from_contracts: remove readonly

[robinkeunen]

This line has the side effect of issuing a warning for all models inheriting resource.mixin and setting defaults to resource_calendar_id.

The readonly was introduced to prevent modifications by the user since the modules computes it. It is however to relevant since the field is hidden (readonly parameter only affects the views).

fixes https://github.com/coopiteasy/addons/issues/311

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5abf60e) 97.71% compared to head (7047976) 97.71%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 16.0 #312 +/- ## ========================================== - Coverage 97.71% 97.71% -0.01% ========================================== Files 26 26 Lines 482 481 -1 Branches 52 52 ========================================== - Hits 471 470 -1 Misses 4 4 Partials 7 7 ```

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