Remove ActiveUserCountThresholdWorker cron schedule

What does this MR do and why?

Removes the cron schedule for ActiveUserCountThresholdWorker to stop new seat limit notification emails from being enqueued. The worker class is kept intact so any already-queued jobs drain safely.

This is the minimal fix to stop the daily emails. A follow-up MR will no-op the worker perform method as part of the full Sidekiq worker removal process.

Related

• Closes #604241 (closed)
• Follow-up to !235870 (merged)

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