fjson and fuzzing cleanup

[AlekSi]

Description

Remove some old code. Fix fuzztool.

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.

[vercel]

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Oct 17, 2022 at 5:45AM (UTC)

[codecov]

Codecov Report

Merging #1262 (86f1b95) into main (68d11f7) will increase coverage by 0.34%.
The diff coverage is 95.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1262      +/-   ##
==========================================
+ Coverage   66.34%   66.69%   +0.34%     
==========================================
  Files         270      270              
  Lines       12609    12614       +5     
==========================================
+ Hits         8366     8413      +47     
+ Misses       3336     3296      -40     
+ Partials      907      905       -2     

Impacted Files	Coverage Δ
internal/bson/bson.go	43.33% <0.00%> (+43.33%)	⬆️
internal/bson/cstring.go	68.18% <ø> (ø)
internal/types/document.go	87.34% <ø> (ø)
internal/bson/document.go	44.96% <100.00%> (+0.93%)	⬆️
internal/util/version/version.go	48.52% <0.00%> (-4.42%)	⬇️
internal/handlers/common/filter.go	87.52% <0.00%> (+0.94%)	⬆️
internal/types/fjson/fjson.go	89.39% <0.00%> (+3.03%)	⬆️
internal/handlers/pg/pgdb/settings.go	44.18% <0.00%> (+4.65%)	⬆️
internal/clientconn/conninfo/conn_info.go	83.33% <0.00%> (+16.66%)	⬆️

Flag	Coverage Δ
integration	62.62% <75.00%> (+0.07%)	⬆️
mongodb	14.13% <40.00%> (-0.01%)	⬇️
pg	52.16% <75.00%> (+0.08%)	⬆️
tigris	33.75% <75.00%> (+0.04%)	⬆️
unit	25.12% <90.00%> (+0.33%)	⬆️

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[chilagrow]

Thanks for the docs!
@AlekSi Fuzz test CI is looking unhappy, doesn't seem like it's related to this PR though https://github.com/FerretDB/FerretDB/actions/runs/3262667842/jobs/5360097653

[AlekSi]

Yeah, it fails due to #364

[chilagrow]

🚀

[rumyantseva]

LGTM

[AlekSi]

Ok, "Fuzz short" is not required for PRs to be merged until we re-add validation and fix all fuzzing issues.