Remove dead _validate_topic function

[lukiod]

Fixes #256.

`_validate_topic` (`src/rosetta-mcp-server/rosetta_mcp/server.py:545`)
has zero call sites in `src/`. Both places that could use it
(`get_context_instructions`, `query_instructions`) hardcode
`topic=None` with an inline comment explaining topic filtering was
intentionally disabled to reduce noise. Leaving the validator in place
implies topic filtering is still active, which is misleading -- removed
per the issue's own preferred option (YAGNI) and the automated triage
comment's recommendation.

Verified: grepped for every reference to `_validate_topic` before and
after -- only the definition existed, confirming this is genuinely dead
code, not something with a hidden caller. Full test suite: 334/334
passing, no regressions.

[github-actions]

Rosetta Triage Review

Summary: Removes the dead _validate_topic function from rosetta_mcp/server.py — it had zero call sites since topic filtering was already hardcoded to None at both call sites. Fixes #256.

Findings:

• Verified via grep across src/ and tests/: no remaining references to _validate_topic after removal.
• Change is minimal (7 deletions, 0 additions), scoped to exactly the dead function, matches the linked issue's preferred fix.
• PR description documents verification steps (grep before/after, full test suite 334/334 passing) and CI checks are green.

Caveats:

• No dedicated test previously covered _validate_topic directly (consistent with it being unreachable), so removal doesn't reduce test coverage.

Automated triage by Rosetta agent

[isolomatov-gd]

Thank you!