posture: stop logging serial numbers

Logging serial numbers every time they are read might have been useful
early on, but seems unnecessary now.

Updates #5902

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

posture/serialnumber_notmacos.go

@@ -98,8 +98,5 @@ func GetSerialNumbers(logf logger.Logf) ([]string, error) {
 			}
 		}
 	}
-
-	logf("got serial numbers %v", serials)
-
 	return serials, nil
 }