NATM: Remove useless NETGRAPH_ATM_ATMPIF option

This code was removed as part of the NATM removal in 2017 and somehow
this option was missed.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D38874

(cherry picked from commit 3746e90118c941eb5c9123e22fe05026b78382ab)