Fix: Use a constant name for the LOAS3 cert watcher.

The periodic call for watching the LOAS3 certificate now uses the dedicated constant `kWatcherForLoas3Cert` as its identifier in the scheduler, rather than the certificate path. This improves clarity and consistency.

Also remove unused const `kUnprovisionedBMC`

PiperOrigin-RevId: 862845107
Change-Id: I8759cde45e2a1e7d682f0f3013d66ffc2790ef38