Remove bogus detach routines

These drivers are not bus drivers, and would need proper detach
routines that tore down state stored in the softc. Better to just
fail detach outright instead of an incorrect success.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D47389

(cherry picked from commit 7ebc7d1ab76b9d06be9400d6c9fc74fcc43603a1)