RFR: 8339237: RISC-V: Builds fail after JDK-8339120

[Gui Cao]

Encountered this build warning/error when doing a native build on linux-riscv64 platform with GCC-13. We can simply remove float_regs_as_doubles_size_in_slots in file c1_Runtime1_riscv.cpp as it is not used anywhere.

### Testing
- [x] release & fastdebug build OK on linux-riscv64 after this change.

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Commit messages:
 - 8339237: RISC-V: Builds fail after JDK-8339120

Changes: https://git.openjdk.org/jdk/pull/20765/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20765&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339237
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20765.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20765/head:pull/20765

PR: https://git.openjdk.org/jdk/pull/20765