Remove agent field option from scan execution docs

What does this MR do and why?

Remove agent field option from scan execution docs

• This MR stemmed from this conversation
• For scan_execution_policies, the cluster field was removed in 15.0 and is being replaced by the agent field
• The agent field is available, but not working yet, so I am removing the agent sections until Extend Scheduled Scan Execution Policy to enfor... (#341358 - closed) is complete

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