[netstack3] Implement fidl_del_forwarding_entry

NET-2105 #done

Testing:
net_cli if add /dev/class/ethernet/000
net_cli fwd add-device 1 192.168.1.0 24
net_cli fwd list                  # Output has one route
net_cli fwd del 192.168.42.0 24   # Error no match
net_cli fwd del 192.168.42.0 16   # Error invalid args
net_cli fwd del 192.168.1.0 24    # Success
net_cli fwd list                  # Output is empty
net_cli fwd del 192.168.1.0 24    # Error no match

Change-Id: I436b9badf4fccbf0637259434105f0636ee35fef