target/riscv: Remove unused include of riscv_htif.h for virt board riscv

target/riscv: Remove unused include of riscv_htif.h for virt board riscv

Signed-off-by: Jonathan Behrens <fintelia@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>