Remove obsolete private language capability constants

[Widthdom]

Remove the ten obsolete private language capability constants and the partial file that contained only them. Capability matching and validation already delegate to LanguageCapabilityCatalog; accepted values, CLI/MCP schemas, and shell completions retain their existing behavior.

Fixes #5262

Verification of removal:

• Each of the ten names was searched individually using the freshly built local cdidx find <name> --all --json --limit 20 --include-generated --db .cdidx/codeindex.db. Every search returned only its declaration with scan_complete: true, authoritative rows, and no truncation.
• A complete broader scan covered capability names and reflection field consumers; none consume these constants. The CLI delegates to the catalog, and the MCP schema obtains values from that catalog.
• Reviewed MSBuild inputs and generation settings: standard SDK compile inclusion, no explicit reference to the deleted file, and no custom generator consuming these private fields.

Validation:

• Post-commit solution build succeeded with zero warnings/errors on both target frameworks.
• Existing language, parser, completion, MCP, and CLI contract tests: net8.0 passed 128/128; net9.0 passed 127, skipped 1, failed 0.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore passed.
• dotnet run --project tools/CodeIndex.Changelog -- check passed.
• Full dotnet test -p:UseSharedCompilation=false is still running, so full-suite success is not yet confirmed. Test/format/review execution required normal permissions because the sandbox blocked local IPC initialization.
• Codex adversarial review of origin/main..HEAD: No blocking/actionable issues found.
• Post-commit root index check reports index_matches_workspace: true; the workspace check reports both members healthy.

PR status: no merge conflicts. Changelog validation and Actions CodeQL passed; the other CI lanes were still pending after three bounded checks.

Documentation: public documentation needs no update because this is behavior-neutral removal of unused private declarations. Added bilingual internal fragment changelog.d/unreleased/5262.internal.md; CHANGELOG.md is unchanged. No source-absence test was added.

Follow-up issues: none identified.