[repo-assist] refactor: remove long-dead commented-out code

[github-actions]

🤖 This PR was created by Repo Assist, an automated AI assistant for this repository.

Summary

Removed several blocks of long-dead commented-out code that had accumulated from earlier refactors, with no live callers and no explanatory TODO tying them to future work:

• GenerateHtml.fs: an unused obsoleteMessage helper (never called; no reference anywhere in the file).
• SymbolReader.fs: four unused signatureTooltip computations (one per member kind: function/value, union case, field, static parameter) and one stray (if/redacted) isUnitType retType then None else Some retTypeText comment — all superseded by the usageHtml/returnType computed nearby, and none read anywhere.
• CodeFormatAgent.fs: an alternate, unused ProjectOptions construction plus stale debug-logging comments referencing a code path that no longer exists.
• HtmlModel.fs: an unused string alias immediately shadowed by the (!!) operator on the very next line.

Rationale

These fragments add noise without adding information — no TODO, issue reference, or design note explains why they're kept. I deliberately left genuinely-explanatory TODO comments alone (e.g. the exceptions/parameters-section TODO in XmlDocReader.fs), since those document real, tracked future work rather than abandoned code.

Trade-offs

None — purely a comment/dead-code removal with no behavior change.

Test Status

• ✅ dotnet build FSharp.Formatting.sln --configuration Release — succeeds, 0 warnings, 0 errors.
• ✅ dotnet test FSharp.Formatting.sln --configuration Release --no-build — full suite passes: 375 (Markdown) + 43 (CodeFormat) + 31 (fsdocs-tool) + 146 (Literate) + 100 (ApiDocs) = 695 passed, 0 failed (6 pre-existing skips unrelated to this change).
• ✅ dotnet fantomas <changed files> --check — no formatting changes needed.

Generated by 🌈 Repo Assist, see workflow run. Learn more.
_{Comment /repo-assist to run again}

_{Add this agentic workflow to your repo}

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@4bc8419fad05e6b032741cbfd189986700bcf71c

[nojaf]

/repo-assist there is no need for a changelog entry for this since this is not user facing

[github-actions]

✓ Repo Assist completed successfully, see workflow run.

Generated by 🌈 Repo Assist, see workflow run. Learn more.