[riscv-port-jdk17u:riscv-port] RFR: 8309427: [riscv-port-jdk17u] Remove unused RoundDoubleModeV C2 node

[Gui Cao]

Hi,
Please help review this backport to riscv-port-jdk17u.
The unused RoundDoubleModeV node has been removed in [JDK-8306966](https://bugs.openjdk.org/browse/JDK-8306966). because the original JBS issue title was not appropriate, so we has been created a new JBS issue.

Testing(release on unmacthed):
- [x] Tier1 tests
- [ ] Tier2 tests
- [ ] Tier3 tests

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Commit messages:
 - 8309427: [riscv-port-jdk17u] Remove unused RoundDoubleModeV node

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/68/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=68&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309427
  Stats: 28 lines in 1 file changed: 0 ins; 28 del; 0 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/68.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/68/head:pull/68

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/68