RFR: 8342579: RISC-V: C2: Cleanup effect of killing flag register for call nodes

[Fei Yang]

Previously found and discussed at: https://github.com/openjdk/jdk/pull/21406#discussion_r1803232561
For C2 call nodes, it's not necessary to add effect listing flag register as being killed.
This cleans them up and thus aligns with other CPU platforms.

Testing on linux-riscv64:
- [x] Tier1 (release build)

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Commit messages:
 - 8342579: RISC-V: C2: Cleanup effect of killing flag register for call nodes

Changes: https://git.openjdk.org/jdk/pull/21576/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21576&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342579
  Stats: 10 lines in 1 file changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/21576.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21576/head:pull/21576

PR: https://git.openjdk.org/jdk/pull/21576