RFR: Re-enable class unloading

[William Kemper]

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.

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Commit messages:
 - Re-enable class unloading

Changes: https://git.openjdk.java.net/shenandoah/pull/55/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=55&range=00
  Stats: 9 lines in 1 file changed: 0 ins; 9 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/55.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/55/head:pull/55

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