Remove superseded internal helpers

[Widthdom]

Summary

• Remove StringRowsEqual, whose callers were superseded by the ordered diff path that already determines equality.
• Remove QuotedIncludeRegex, because the active shader IncludeRegex handles both quoted and angle-bracket includes.
• Remove HasPrologClauseTerminator and PreferDefinitionAtPosition, whose call paths now use the top-level Prolog terminator scanner and direct position-aware definition resolution.
• Add the required bilingual internal changelog fragment.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• Focused behavior tests: 8/8 passed on both net8.0 and net9.0
• Full net8.0 suite: 11,085 passed, 7 skipped, 0 failed
• Full net9.0 suite: 10,611 passed, 420 skipped, 0 failed
• Changelog validator: 93 fragments validated
• Local index freshness: 1,333/1,333 files matched; all four removed names return 0 exact matches
• Codex adversarial review: no blocking or actionable findings

Documentation / changelog

• Added changelog.d/unreleased/4917.internal.md in English and Japanese.
• No public documentation change is needed because this removes unreachable private helpers without changing behavior.

Follow-up candidates

• None.

Fixes #4917