RFR: 8345040: Clean up unused variables and code in `generate_native_wrapper`

[Qizheng Xing]

Some of variables and code are related to critical JNI natives feature, which was removed in JDK 18. This patch cleans them up.

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Commit messages:
 - Remove unused variable `after_transition` in `generate_native_wrapper`.
 - Remove unused variable `tmp_vmreg` and `temploc` in `generate_native_wrapper`.
 - Remove unused variable `in_elem_bt` in `generate_native_wrapper`.

Changes: https://git.openjdk.org/jdk/pull/22384/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22384&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345040
  Stats: 29 lines in 6 files changed: 0 ins; 29 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22384.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22384/head:pull/22384

PR: https://git.openjdk.org/jdk/pull/22384