ifnet: Remove unreachable code

The ioctls SIOCSIFVNET and SIOCSIFRVNET are for userland only. For
SIOCSIFVNET, if_vmove_loan(), the interface is obtained from current
VNET. For SIOCSIFRVNET, if_vmove_reclaim(), a valid child prison is
held before getting the interface. In both cases the VNET of the
obtained interfaces is stable, so there's no need to check it.

No functional change intended.

Reviewed by: glebius, jamie (for Jails)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D55828