pool: drop per-query connection log

[avikivity]

The pool module emits a DEBUG log message when selecting a connection for a query.

Emitting a log message for every query is too noisy.

Since Python logging lacks a TRACE level, just remove the log.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[Copilot]

Pull request overview

This PR reduces debug log noise in the connection pool by removing a per-query DEBUG message emitted when selecting a shard-aware connection, which can be extremely chatty under normal query load.

Changes:

• Removed the per-query log.debug(...) call in _get_connection_for_routing_key() when a shard-specific connection is chosen.

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