target/riscv: Remove RVB (replaced by Zb[abcs])

With everything classified as Zb[abcs] and pre-0.93 draft-B
instructions that are not part of Zb[abcs] removed, we can remove the
remaining support code for RVB.

Note that RVB has been retired for good and misa.B will neither mean
'some' or 'all of' Zb*:
  https://lists.riscv.org/g/tech-bitmanip/message/532

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20210911140016.834071-16-philipp.tomsich@vrull.eu
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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