Code review comment for ~sylvain-pineau/checkbox-support:discard_usb_disks

This MR works well for the latest SRU test. The item "disk/read_performance_sdb", which is actually a usb stick, goes away.

[1] disk/read_performance_sdb test untested job cannot be started: resource expression "block_device.sdb_state != 'removable'" evaluates to false