net: net_pkt: Simplify net_pkt_compact() API

Since db11fcd1 [net/net_pkt: Fully
separate struct net_pkt from struct net_buf], net_pkt_compact() can not
fail anymore.

This commit acknowledges this and simplifies the API accordingly.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>