Cleanup pgdb SQL statements

[w84thesun]

Description

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Sep 29, 2022 at 3:57PM (UTC)

[codecov]

Codecov Report

Merging #1193 (14e304c) into main (fb31975) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1193      +/-   ##
==========================================
- Coverage   66.35%   66.33%   -0.03%     
==========================================
  Files         251      251              
  Lines       12293    12293              
==========================================
- Hits         8157     8154       -3     
- Misses       3246     3248       +2     
- Partials      890      891       +1     

Impacted Files	Coverage Δ
integration/setup/setup.go	87.25% <ø> (ø)
integration/setup/setup_compat.go	88.39% <ø> (ø)
internal/handlers/pg/pgdb/settings.go	39.53% <100.00%> (ø)
internal/util/version/version.go	55.00% <0.00%> (-5.00%)	⬇️
internal/handlers/common/filter.go	86.58% <0.00%> (ø)

Flag	Coverage Δ
integration	62.62% <100.00%> (-0.03%)	⬇️
mongodb	14.89% <0.00%> (-0.01%)	⬇️
pg	51.86% <100.00%> (-0.03%)	⬇️
tigris	34.67% <0.00%> (-0.03%)	⬇️
unit	24.00% <100.00%> (-0.03%)	⬇️

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