Remove stale TechDocs annotation from metadata.yaml

[ChrisMera]

Summary

• Removes the leftover backstage.io/techdocs-ref: dir:. annotation (and the now-empty annotations: block) from metadata.yaml. TechDocs was added in rshakespear/speed-3253-techdocs-guardrail #170 and reverted in Revert "rshakespear/speed-3253-techdocs-guardrail" #171, but this annotation was left behind, so guardrail still shows up in the Backstage Docs index despite having no docs.
• Repo-wide search confirms no other TechDocs/mkdocs remnants (no mkdocs.yml, no CI build steps). docs/plugins.md was reviewed and kept — it's genuine plugin documentation linked from the README, not an mkdocs artifact.

Notes

• After merge, the Backstage catalog picks this up on its regular sync; to see it immediately, hit refresh on the guardrail entity's About card in Backstage.
• Cleaning up stale built docs in the TechDocs S3 bucket is out of scope.

Test plan

• metadata.yaml parses as valid YAML; kind/name/spec fields unchanged
• grep -i techdocs returns nothing repo-wide

[ChrisMera]

Superseded by #173 (same change, moved to branch cmera/SPEED-3390).