RFR: 8367576: JvmtiThreadState::_debuggable is unused

[Francesco Andreuzzi]

The flag has been there since 8153779ad32d1e8ddd37ced826c76c7aafc61894, but seems to be unused.

Testing: `tier1`.

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Commit messages:
 - remove

Changes: https://git.openjdk.org/jdk/pull/27265/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27265&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367576
  Stats: 10 lines in 2 files changed: 0 ins; 10 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27265.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27265/head:pull/27265

PR: https://git.openjdk.org/jdk/pull/27265