KVM: riscv: selftests: Drop SBI multi registers

These registers are no longer getting added to get-reg-list.
We keep sbi_ext_multi_id_to_str() for printing, even though
we don't expect it to normally be used, because it may be
useful for debug.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>