sadf: PCP: Don't check for newly registered network interfaces

sadf: PCP: Don't check for newly registered network interfaces

It is useless to check if current network interface has just been
registered or not before displaying its PCP statistics.
This is because PCP displays the raw counter values, not their variation
over the interval. Moreover the whole list of network interfaces present
in file has been created with each one having already its instance.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>