target/riscv: remove kvm-stub.c

target/riscv: remove kvm-stub.c

This file is not needed for some time now. Both kvm_riscv_reset_vcpu()
and kvm_riscv_set_irq() have public declarations in kvm_riscv.h and are
wrapped in 'if kvm_enabled()' blocks that the compiler will rip it out
in non-KVM builds.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20230925175709.35696-11-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>