net: remove stale altq_input reference

Code setting it was removed in:
commit 325fab802e1f40c992141f945d0788c0edfdb1a4
Author: Eric van Gyzen <vangyzen@FreeBSD.org>
Date: Tue Dec 4 23:46:43 2018 +0000

altq: remove ALTQ3_COMPAT code

Reviewed by: glebius, kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D36471