config: Make ISAPNP be in opt_dontuse.h

Nothing uses ISAPNP today, apart from bringing in files or not. There's
really no need to ever do #ifdef ISAPNP in drivers and such. It means
use the ISA bus plug and play isolation protocol to enumerate the bus,
not the more useful 'you might have devices with isa pnp ids' which all
drivers hide behind DEV_ISA and/or an isa clause in the files files.

Sponsored by: Netflix
Reviewed by: kevans, emaste
Differential Revision: https://reviews.freebsd.org/D37109