[DEV_HANDLER][DOCS] Remove mention of PINGs being sent by Lowcar

[levincent06]

The only messages that Lowcar sends to Dev Handler are ACK, DEVICE_DATA, and LOG. Lowcar used to also send PINGs, but Dev Handler considers any incoming message as an online status message.

There may be some places in the code and documentation that suggest that dev handler can receive PINGs from Lowcar.

[levincent06]

This turned out to be a little more difficult than I thought because the virtual devices still send PINGs to dev handler. (Real Lowcar devices don't send PINGs.) Removing virtual device PINGs causes some devices to time out if more than 5 or so virtual devices try to connect.

Unsure why this is happening... my guess is that since each virtual device is a different process, the context switching is too slow and makes dev handler think the virtual devices time out.