RFR: 8301346: Remove dead emit_entry_barrier_stub definition

[Vladimir Kozlov]

On Mon, 30 Jan 2023 13:27:20 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> [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

Tes, it is trivial.

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Marked as reviewed by kvn (Reviewer).

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