Remove unnecessary Clickhouse connection string validation rule

What does this MR do and why?

It's no longer possible ot set the clickhouse_connection_string application setting, so we should no longer validate for its presence. The field is now called product_analytics_clickhouse_connection_string which has its own validation.

Relates to Create encrypted application setting for produc... (!107264 - merged)

Screenshots or screen recordings

Error state because of validation rule for a field that doesn't exist, e.g., clickhouse_connection_string

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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