Stop printing to stderr in lightning-net-tokio for disconnections

It isn't exactly a critical error situation when we disconnect a
socket, so we shouldn't be printing to stderr, entirely bypassing
user logging, when it happens. We do still print to stderr if we
fail to write the first message to the socket, but this should
never happen unless the user has a reasonably-configured system
with at least one packet in bytes available for the socket buffer.