RFR: JDK-8314389: AttachListener::pd_set_flag obsolete

[Matthias Baesken]

AttachListener::pd_set_flag is the same across platforms (always returning JNI_ERR ). So it can be centralized or removed.

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

Commit messages:
 - JDK-8314389

Changes: https://git.openjdk.org/jdk/pull/15304/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15304&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314389
  Stats: 27 lines in 6 files changed: 0 ins; 26 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15304.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15304/head:pull/15304

PR: https://git.openjdk.org/jdk/pull/15304