ifnet: Remove dead code
Since change [1], if_bpf will not be detached by the interface departure
eventhandler and will not be NULL. Then the logic to re-attach if_bpf
becomes dead and serves no purpose any more.
This partially reverts commit 05fc416403ec.
- 9ce40d321dd5 bpf: Fix incorrect cleanup
Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45599