RFR: 8259583: Remove unused decode_env::_codeBuffer

[Aleksey Shipilev]

SonarCloud instance reports that decode_env::_codeBuffer field is not initialized in constructor. But really, the field is never used after [JDK-8222079](https://bugs.openjdk.java.net/browse/JDK-8222079).

Additional testing:
 - [x] No grep hits for `_codeBuffer` in `src/`

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Commit messages:
 - 8259583: Remove unused decode_env::_codeBuffer

Changes: https://git.openjdk.java.net/jdk/pull/2034/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2034&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259583
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2034.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2034/head:pull/2034

PR: https://git.openjdk.java.net/jdk/pull/2034