iflib(9): Remove support for cloning pseudo interfaces

This code was used by the first incarnation of wg(4) and is dead ever
since f187d6dfbf633665ba6740fe22742aec60ce02a2 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

Reviewed by: erj, kbowling
Differential Revision: https://reviews.freebsd.org/D41196