x86: Remove invalid DEVMETHOD methods for leaf devices

None of these drivers are for bus devices, so bus_generic_* is not
appropriate. Most of these were nops except that detach would
actually "succeed" (but not do any cleanup).

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D47374

(cherry picked from commit df615735960370255d3acc4ac2a6f4fd297b7461)