ifnet: Make if_detach_internal() and if_vmove() void

if_detach_internal() never fail since change [1]. As a consequence,
also does its caller if_vmove(). While here, remove a stall comment.

No functional change intended.

This reverts commit c7bab2a7ca9a6dae79f970c6730a19b65a1ed86e.

[1] a779388f8bb3 if: Protect V_ifnet in vnet_if_return()

Reviewed by: glebius
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D48820