hifn/safe: Remove dummy calls to bus_generic_detach

These drivers do not add child devices, so there's no need to try to
detach children.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D47390

(cherry picked from commit 8b54d874c70d9fb3ed2e059f65c91b3e0eaadbd7)