iflib: Fix detach of pseudo interfaces

In commit 38bfc6dee33b we added an IFDI_DETACH() call to
iflib_pseudo_deregister() since it looked like it was missing. One is
present in the error-handling path of iflib_pseudo_register(). However,
the detach actually comes from the DEVICE_DETACH() method for the
above-mentioned device_t, so now we're calling IFDI_DETACH() twice when
destroying a pseudo interface.

Fix the problem by not calling IFDI_DETACH() from the device detach
routine. This way we can ensure that iflib de-initialization always
happens in a consistent order. It also ensures that you can't do silly
things like "devctl detach <pseudo ifnet>", which would previously
detach the driver without tearing down the corresponding ifnet.

PR: 253541
Reviewed by: erj
MFC after: 1 week
Fixes: 38bfc6dee33b
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28774