riscv: Remove old qemu compatibility code

During early qemu development, the /soc node was marked as compatible
with "riscv-virtio-soc" instead of "simple-bus".

This was changed in qemu 53f54508dae6 in Sep 2018, and predates the
baseline required qemu version (5.0) for riscv by a wide margin.

The generic simplebus code handles attachment in all cases nowadays.

Sponsored by: Tag1 Consulting, Inc.
Reviewed by: jrtc27, mhorne
Differential Revision: https://reviews.freebsd.org/D30011