pfsync: remove invalid panic

When we undefer a packet (when the peer acks the state) it's possible
that we don't find a corresponding pfsync_deferral. We panic here, but
that's actually something that can happen in normal operation:

• if we have too many deferred packets already (in pfsync_defer())
• if the deferral timed out (in pfsync_defer_tmo())

Remove this panic and document the scenarios where it might occur.

MFC after: 2 weeks
Sponsored by: Orange Business Services

(cherry picked from commit 035e87247f845500b4672e10efb8f47fd2c0f2a2)