RFR: Re-enable class unloading

[Roman Kennke]

On Mon, 9 Aug 2021 23:48:04 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Class unloading was disabled for generational mode for the reasons described in [PR 50](https://github.com/openjdk/shenandoah/pull/50). However, the code has since evolved to avoid using `oop::size` when filling in dead objects so we no longer require classes for unreachable objects to remain loaded.

Marked as reviewed by rkennke (Lead).

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PR: https://git.openjdk.java.net/shenandoah/pull/55