RFR: 8254559: Remove unimplemented JVMFlag::get_locked_message_ext

[Aleksey Shipilev]

The definition was removed with JDK-8224599, the declaration was left behind.

Testing:
 - [x] Linux x86_64 build
 - [x] Text search for `get_locked_message_ext` in entire `src/`

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Commit messages:
 - 8254559: Remove unimplemented JVMFlag::get_locked_message_ext

Changes: https://git.openjdk.java.net/jdk/pull/598/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=598&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254559
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/598.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/598/head:pull/598

PR: https://git.openjdk.java.net/jdk/pull/598