RFR: 8339298: Remove unused function declaration poll_for_safepoint

[Fei Yang]

On Fri, 30 Aug 2024 07:03:57 GMT, Gui Cao <gcao at openjdk.org> wrote:

> 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

Looks good. I find this is leftover for 
[8189596: AArch64: implementation for Thread-local handshakes](https://bugs.openjdk.org/browse/JDK-8189596)

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Marked as reviewed by fyang (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20785#pullrequestreview-2271441289