hw/riscv: virt: Remove the redundant ipi-id property

hw/riscv: virt: Remove the redundant ipi-id property

The imsic DT binding[1] has changed and no longer require an ipi-id.
The latest IMSIC driver dynamically allocates ipi id if slow-ipi
is not defined.

Get rid of the unused dt property which may lead to confusion.

[1] https://lore.kernel.org/lkml/20221111044207.1478350-5-apatel@ventanamicro.com/

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221122080529.1692533-1-atishp@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>