p2p: silence on listener shutdown

[jsvisa]

When running with debug log(--verbosity 5), it will always print the below log just before the node shutdown:

INFO [10-21|15:23:28.155] Blockchain stopped
TRACE[10-21|15:23:28.184] P2P networking is spinning down
DEBUG[10-21|15:23:28.184] Read error                               err="accept tcp [::]:30303: use of closed network connection"

The Read error msg is annoying, so propose to remove it.