Rust 2021: Remove new prelude uses

src/arch/aarch64/mm/physicalmem.rs

src/arch/aarch64/mm/virtualmem.rs

src/arch/x86_64/kernel/apic.rs

src/arch/x86_64/kernel/mod.rs

src/arch/x86_64/kernel/pci.rs

src/arch/x86_64/kernel/processor.rs

src/arch/x86_64/kernel/scheduler.rs

src/arch/x86_64/mm/paging.rs

src/arch/x86_64/mm/physicalmem.rs

src/arch/x86_64/mm/virtualmem.rs

src/drivers/net/rtl8139.rs

src/drivers/net/virtio_net.rs

src/drivers/virtio/depr/virtio.rs

src/drivers/virtio/env.rs

src/drivers/virtio/transport/pci.rs

src/drivers/virtio/virtqueue/packed.rs

src/drivers/virtio/virtqueue/split.rs

src/scheduler/mod.rs

src/scheduler/task.rs

src/syscalls/interfaces/mod.rs