See if `VirtualAdapter` really needs to periodically send scan_events

VirtualAdapter, when first written, sent periodic scan events for all virtual devices attached to it. This was important because CoreTools didn't yet support `probing` for devices and required that DeviceAdapter subclasses push events for each device seen.

We have since impemented probing and VirtualAdapter uses probing now but it still sends gratuitous scan events in its periodic adapter callback. We should verify if this is actually still necesary or just leftover from an incomplete refactor.

The place where it could still be necessary is if IOTileGateway does not support probing yet so this behavior is necessary for VirtualAdapters to work when connected to an IOTileGateway.