RFR: 8301346: Remove dead emit_entry_barrier_stub definition

[Tobias Hartmann]

[JDK-8297036](https://bugs.openjdk.org/browse/JDK-8297036) removed the implementation of `emit_entry_barrier_stub` and `entry_barrier_stub_size` but not the definition in the header files.

Thanks,
Tobias

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Commit messages:
 - 8301346: Remove dead emit_entry_barrier_stub definition

Changes: https://git.openjdk.org/jdk/pull/12294/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12294&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301346
  Stats: 18 lines in 6 files changed: 0 ins; 18 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12294.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12294/head:pull/12294

PR: https://git.openjdk.org/jdk/pull/12294