RFR: 8339298: Remove unused function declaration poll_for_safepoint

[Gui Cao]

Hi, I noticed that there are two unused function declarations here, in the historical version they were used without UseCompilerSafepoints, now the unused UseCompilerSafepoints have been removed, but the function declarations may have forgotten to be removed.

### Testing
- [x] release & fastdebug build OK on linux-aarch64
- [x] release & fastdebug build OK on linux-riscv64

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Commit messages:
 - 8339298: Remove unused function declaration poll_for_safepoint

Changes: https://git.openjdk.org/jdk/pull/20785/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20785&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339298
  Stats: 4 lines in 2 files changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20785.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20785/head:pull/20785

PR: https://git.openjdk.org/jdk/pull/20785