Remove done TODOs

[AlekSi]

Description

Closes #3742.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3795 (94393b0) into main (e073f9d) will decrease coverage by 0.07%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3795      +/-   ##
==========================================
- Coverage   76.32%   76.25%   -0.07%     
==========================================
  Files         315      315              
  Lines       23506    23506              
==========================================
- Hits        17940    17925      -15     
- Misses       4524     4538      +14     
- Partials     1042     1043       +1     

Files	Coverage Δ
internal/backends/collection.go	92.62% <ø> (ø)

... and 10 files with indirect coverage changes

Flag	Coverage Δ
filter-true	72.40% <ø> (-0.07%)	⬇️
hana-1	?
integration	72.40% <ø> (-0.07%)	⬇️
mongodb-1	5.36% <ø> (ø)
postgresql-1	51.36% <ø> (ø)
postgresql-2	52.69% <ø> (-0.12%)	⬇️
postgresql-3	52.15% <ø> (+<0.01%)	⬆️
sqlite-1	50.60% <ø> (ø)
sqlite-2	51.91% <ø> (+0.06%)	⬆️
sqlite-3	51.20% <ø> (-0.04%)	⬇️
unit	30.73% <ø> (ø)

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