Drop ScenarioActiveFlag import and export mappings

[soininen]

The flag is not used nor accessible to users.

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 44.44444% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 78.24%. Comparing base (5736d72) to head (afc41dc).

Files	Patch %	Lines
spinedb_api/export_mapping/export_mapping.py	0.00%	1 Missing and 1 partial :warning:
spinedb_api/import_mapping/import_mapping.py	66.66%	1 Missing and 1 partial :warning:
spinedb_api/export_mapping/settings.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #400 +/- ## ========================================== - Coverage 78.29% 78.24% -0.06% ========================================== Files 76 76 Lines 9514 9501 -13 Branches 2012 2013 +1 ========================================== - Hits 7449 7434 -15 Misses 1734 1734 - Partials 331 333 +2 ```

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