hw/riscv/riscv-iommu: Remove redundant struct members

hw/riscv/riscv-iommu: Remove redundant struct members

Initially, the IOMMU would create a thread, but this thread was removed in
the merged version. The struct members for thread control should have been
removed as well, but they were not removed in commit 0c54acb8243
("hw/riscv: add RISC-V IOMMU base emulation").

Signed-off-by: Jason Chien <jason.chien@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250115141730.30858-1-jason.chien@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>