[riscv-port-jdk17u] Remove unused RoundDoubleModeV C2 node

Hi, here we want to remove the RoundDoubleModeV node that is not being used, we found that the reason the RoundDoubleModeV node is not called in RISC-V is that the PopulateIndex node needs to be added to enable SuperWord optimization (an algorithm for automatic vectorization). In addition to this, the node RoundDoubleMode needs to be implemented in its scalar form, which is not implemented in riscv.ad. But there is no floating point to floating point instruction in riscv that does not change the width by, so what we need to do here may simply be to remove the RoundDoubleModeV node. refer: https://bugs.openjdk.org/browse/JDK-8298342