RFR: 8274927: Remove unnecessary G1ArchiveAllocator code

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that removes code that is fairly trivially recognizable as useless?

The `_archive_allocator` is always freed after use (see `HeapShared::copy_closed/open_objects`), so it must always be `nullptr` when it's accessed outside the CDS dump code.
I kept the asserts in the original places, but idk if that is really necessary.

Testing: tier1-5

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Commit messages:
 - Improved log messages
 - Cleanup unnecessary archive allocator stuff

Changes: https://git.openjdk.java.net/jdk/pull/5861/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5861&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274927
  Stats: 36 lines in 3 files changed: 0 ins; 26 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5861.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5861/head:pull/5861

PR: https://git.openjdk.java.net/jdk/pull/5861