chore(sites): drop FlexJobs

FlexJobs is pay-to-view (jobseekers must subscribe to see actual
listings) — scam-adjacent, not a legitimate discovery source.
Independently, the smartextract crawler couldn't reach the site
anyway (Akamai IP-block from this network), so removing it has no
discovery cost: zero FlexJobs rows currently in the DB.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>