RFR: 8320877: Shenandoah: Remove ShenandoahUnloadClassesFrequency support

[Roman Kennke]

On Tue, 28 Nov 2023 12:05:16 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Flag cleanup. `ShenandoahUnloadClassesFrequency` was added long time ago when Shenandoah did not support concurrent class unloading. We kept it in to simplify backports. At this point, most releases support concurrent class unloading, and there is no reason to keep the flag in. The default is "1" since [JDK-8228720](https://bugs.openjdk.org/browse/JDK-8228720).
> 
> Additional testing:
>  - [x] MacOS AArch64 server fastdebug, `hotspot_gc_shenandoah`

Makes sense and looks good, thank you!

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Marked as reviewed by rkennke (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16847#pullrequestreview-1755822674