This parameter is no longer needed for the reconciler sync command.
Removed all related functionality including:

- CLI parameter definition in Sync.get_parser()
- flush_cache variable assignment in Sync.take_action()
- flush_cache parameter from reconciler.run() function
- FLUSH_CACHE environment variable logic

Depends-on: https://github.com/osism/container-image-inventory-reconciler/pull/458

AI-assisted: Claude Code

Signed-off-by: Christian Berendt <berendt@osism.tech>