Do not fetch from folders other than Spam during scanning

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On Gmail virtual Sent folder is not correctly detected as virtual and is not skipped, resulting in message ordering issues: #7178 (comment)

Rather than trying to fix virtual folder detection simpler solution is to stop scanning folders other than Spam. Scanning of other folders was introduced without a good reason, the original problem was that contact requests were sent to spam, see #1994

This issue is a replacement for discussion in #7187