if_bnxt: Remove sysctl node vlan_only which is not being used

Removed sysctl node vlan_only which is not being used after migrating
from l2_set_rx_mask to l2_filter_alloc.

Reviewed by: imp
Differential Revsion: https://reviews.freebsd.org/D36444

(cherry picked from commit 72e9dbb58cad5262190cf2eae47f764021072128)