Fix internal server error in timeslot admin due to removed method

[johannaengland]

This is a leftover from moving from "Django MultiSelectField" to "ArrayField". The method get_days_list() was specific to "Django MultiSelectField".

Important PR, because currently the page /admin/argus_notificationprofile/timeslot/ fails with an internal server error.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 84.63%. Comparing base (6488afa) to head (526a8bd).

:exclamation: Current head 526a8bd differs from pull request most recent head e1ed24d. Consider uploading reports for the commit e1ed24d to get more accurate results

Files	Patch %	Lines
src/argus/notificationprofile/models.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #797 +/- ## ========================================== + Coverage 84.61% 84.63% +0.02% ========================================== Files 75 75 Lines 3750 3749 -1 ========================================== Hits 3173 3173 + Misses 577 576 -1 ```

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Test results

       7 files     574 suites   21m 35s :stopwatch:    462 tests    461 :heavy_check_mark: 1 :zzz: 0 :x: 3 234 runs  3 227 :heavy_check_mark: 7 :zzz: 0 :x:

Results for commit e1ed24dd.