Fix issue when readding a committed, then removed item

[soininen]

This PR fixes a bug where a committed item would disappear from the DB if it was removed, then readded (as new item, not via restore()).

Fixes spine-tools/Spine-Toolbox#2743

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 89.65517% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 78.16%. Comparing base (02807aa) to head (c7ed0a3).

Files	Patch %	Lines
spinedb_api/db_mapping.py	81.81%	1 Missing and 1 partial :warning:
spinedb_api/db_mapping_base.py	94.44%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #408 +/- ## ========================================== - Coverage 78.28% 78.16% -0.13% ========================================== Files 76 72 -4 Lines 9543 9515 -28 Branches 2025 2031 +6 ========================================== - Hits 7471 7437 -34 - Misses 1737 1740 +3 - Partials 335 338 +3 ```

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