RFR: 8273590: Move helper classes in G1 post evacuation sub tasks to cpp files

[Thomas Schatzl]

Hi all,

  can I have reviews for this straightforward refactoring, moving helper classes that are not required to .cpp files. The nice side effect of this is that it removes around 100 LOC. No changes to code, except for removal of one unused local variable in
```RemoveSelfForwardPtrsTask::~RemoveSelfForwardPtrsTask`.

Testing: compilation, gha, local jtreg/gc/g1

Thanks,
  Thomas

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Commit messages:
 - Move post evacuate helper classes to cpp file

Changes: https://git.openjdk.java.net/jdk/pull/5461/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5461&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273590
  Stats: 423 lines in 2 files changed: 108 ins; 190 del; 125 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5461.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5461/head:pull/5461

PR: https://git.openjdk.java.net/jdk/pull/5461