RFR: 8334430: Clean up nativeInst_x86.*

Tue Jun 18 01:04:35 UTC 2024

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

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Commit messages:
 - 8334430: Clean up nativeInst_x86.*

Changes: https://git.openjdk.org/jdk/pull/19758/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19758&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334430
  Stats: 331 lines in 2 files changed: 0 ins; 329 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19758.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19758/head:pull/19758

PR: https://git.openjdk.org/jdk/pull/19758