[repo-assist] Remove dead code in TakeCodeBlock active pattern

[github-actions]

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

What

Removes dead code from the (|TakeCodeBlock|_|) active pattern in src/Common/StringParsing.fs:

1. A stale, fully commented-out implementation block (lines that computed nothing and were never active).
2. A tautological guard condition spaceNum >= 4 in the pattern match — spaceNum is a local let hardcoded to 4 a few lines above and never varies, so this condition always evaluated to true and was misleading (it reads as if spaceNum could take other values).

Why

Pure readability cleanup with no behavior change: the guard always passed, so removing it changes nothing about parsing results. The commented-out block was leftover from an earlier draft of the implementation and added noise without value.

Trade-offs

None — this is a strictly internal, non-breaking cleanup confined to a single private active pattern. No public API, no user-visible behavior, and no RELEASE_NOTES entry is needed per project convention (internal-only change).

Test Status

• ✅ dotnet build FSharp.Formatting.sln --configuration Release — build succeeded, 0 warnings, 0 errors.
• ✅ dotnet test tests/FSharp.Markdown.Tests --configuration Release --no-build — 375/375 tests passed.
• ✅ dotnet fantomas src/Common/StringParsing.fs — 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