RFR: 8344265: RISC-V: Remove unused function get_previous_sp_entry

[Gui Cao]

Hi, I noticed that there are a several unused functions here that are currently only used in the x86 architecture, not used in RISC-V.

### Testing
 - [x] release & fastdebug cross-build  linux-riscv64 OK
 - [x] release & fastdebug build  on SOPHON SG2042
- [ ] Run tier1 tests on SOPHON SG2042 (release)

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Commit messages:
 - 8344265: RISC-V: Remove unused function get_previous_sp_entry

Changes: https://git.openjdk.org/jdk/pull/22130/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22130&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344265
  Stats: 58 lines in 2 files changed: 0 ins; 58 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22130.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22130/head:pull/22130

PR: https://git.openjdk.org/jdk/pull/22130