altq: remove ALTQ3_COMPAT code

This code has apparently never compiled on FreeBSD since its
introduction in 2004 (r130365). It has certainly not compiled
since 2006, when r164033 added #elsif [sic] preprocessor directives.
The code was left in the tree to reduce the diff from upstream (KAME).
Since that upstream is no longer relevant, remove the long-dead code.

This commit is the direct result of:

unifdef -m -UALTQ3_COMPAT sys/net/altq/*

A later commit will do some manual cleanup.

I do not plan to MFC this. If that would help you, go for it.