net: ping6: remove a pr_debug() statement

We have ftrace and BPF today, there's no need for printing arguments
at the start of a function.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>