RFR: 8334430: Clean up nativeInst_x86.*

[Jie Fu]

On Tue, 18 Jun 2024 00:59:52 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

> Remove code in `nativeInst_x86.*` added by [JDK-8171008](https://bugs.openjdk.org/browse/JDK-8171008) old JAOTC implementation and currently unused after JAOTC code was removed.
> Aarch64 code was already removed by an other cleanup [JDK-8333410](https://bugs.openjdk.org/browse/JDK-8333410).
> 
> I also removed `NativePopReg` class which was added during initial HotSpot development and never used.
> 
> Tested: tier1

LGTM

-------------

Marked as reviewed by jiefu (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19758#pullrequestreview-2125670777