do not keep deleted annotations in ram

[Eldies]

Motivation and context

When annotations are imported into a job, previous annotations are deleted. But after deletion they are kept in memolry for no reason, which can be a problem if they are large.
Dont keep deleted annotations in memory anymore.

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.16%. Comparing base (2f110e5) to head (910a3e4).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9256      +/-   ##
===========================================
- Coverage    74.17%   74.16%   -0.01%     
===========================================
  Files          428      428              
  Lines        44964    44968       +4     
  Branches      3917     3917              
===========================================
  Hits         33352    33352              
- Misses       11612    11616       +4     

Components	Coverage Δ
cvat-ui	77.21% <ø> (-0.02%)	⬇️
cvat-server	71.63% <90.00%> (+<0.01%)	⬆️

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