target/riscv: Remove privilege v1.9 specific CSR related code

target/riscv: Remove privilege v1.9 specific CSR related code

Qemu doesn't support RISC-V privilege specification v1.9. Remove the
remaining v1.9 specific references from the implementation.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20210319194534.2082397-2-atish.patra@wdc.com>
[Changes by AF:
 - Rebase on latest patches
 - Bump the vmstate_riscv_cpu version_id and minimum_version_id
 ]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>