Remove code related to scenario active flag import/export

[soininen]

The mapping is now gone from spinedb_api

Re spine-tools/Spine-Toolbox#2738

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 62.67%. Comparing base (f146f6a) to head (0be3452).

Files	Patch %	Lines
spine_items/importer/mvcmodels/mappings_model.py	0.00%	2 Missing :warning:
...ms/exporter/widgets/specification_editor_window.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #209 +/- ## ========================================== - Coverage 62.70% 62.67% -0.03% ========================================== Files 190 190 Lines 17908 17909 +1 Branches 2952 2951 -1 ========================================== - Hits 11229 11225 -4 - Misses 6076 6080 +4 - Partials 603 604 +1 ```

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