Delete unused threading.html documentation (use only Markdown file)

[chrjohn]

The new documentation for the QuickFIX/J threading model should exist only as docs/threading-model.md in GitHub markdown format.

Please remove the duplicate HTML version (quickfixj-core/src/main/doc/usermanual/usage/threading.html). Only the markdown should remain and be maintained going forward, as it is sufficient for both contributors and users.

Checklist:

• Ensure docs/threading-model.md exists and is up to date
• Remove quickfixj-core/src/main/doc/usermanual/usage/threading.html if present
• Update any README or user manual index references to point only to the markdown documentation, if applicable