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()
MFC after: 3 days