docs/system: Move the RISC-V -bios information to removed

QEMU 5.1 changed the behaviour of the default boot for the RISC-V virt
and sifive_u machines. This patch moves that change from the
deprecated.rst file to the removed-features.rst file and the
target-riscv.rst.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 4f1c261e7f69045ab8bb8926d85fe1d35e48ea5b.1620081256.git.alistair.francis@wdc.com