[18.4] Revert discarded database pool check in load balancer

What does this MR do and why?

This backports !207900 (merged) for 18-4-stable-ee.

Some users have reported intermittent No connection pool for 'ActiveRecord::Base' found errors after updating to GitLab 18.4. This is likely due to !203779 (merged) because the QueryAnalyzer is likely accessing the database before the database load balancer has properly initialized everything.

Since the related issue was solved with !203888 (merged), let's revert the discarded? check for now.

Relates to #573581 (closed)

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