inpcb: Remove an incorrect assertion in in_pcblbgroup_find()

The inpcb might indeed not belong to a load-balancing group when passed
from in_pcblbgroup_numa().

Reported by: syzbot+58bb60fe7a452df78835@syzkaller.appspotmail.com
Fixes: 7cbb6b6e28db ("inpcb: Close some SO_REUSEPORT_LB races, part 2")