iflib(9): Remove support for cloning pseudo interfaces

[marius]

Summary

The iflib(9) code for cloning pseudo interfaces was used by the first
incarnation of wg(4) and is dead ever since f187d6dfbf63 has removed
the latter again. Moreover, this code matched iflib(4) like a square
peg fits in a round hole, was incomplete and despite some hacks still
tailored to VPC and wg(4) but not generic. In effect, this reverts the
following:
09f6ff4f1a47c3009dc16fdc609a44f2341bc7ac (w/ its "ancillary changes")
9aeca21324f481f57f2ecb7009f461f4f51b62b3
1f93e931d9f0c688f43f98ef777e04636a325526
0f9544d03e89d180f94a7a84b110ec7d2b6c625a
0dd691b41276ce13d25ffb1443af27f85038aa3f

Test Plan

o Compile tested in a universe build
o Run-time tested with EM- and IGB-class hardware

[erj]

I like the idea, and I don't see anything that looks wrong after looking through it quickly. We've added support to ice(4) internally to allow for an extra interface for traffic mirroring that is attached to the main interface, but we're using the standard iflib device interfaces to create it and manipulate it instead of the pseudo interfaces, so this won't affect that.

[marius]

Close, committed as 7ff9ae90f0bd133f56930858bb478a309da3982b w/ fat-fingered URL