RFR: 8356756: Cleanup: Make ClassLoaderData::_deallocate_list an object

[Johan Sjölen]

A small cleanup in order to get rid of some branches checking for null.

Testing: GHA

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Commit messages:
 - Make ClassLoaderData::_deallocate_list an ordinary object

Changes: https://git.openjdk.org/jdk/pull/25117/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25117&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356756
  Stats: 30 lines in 2 files changed: 0 ins; 15 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/25117.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25117/head:pull/25117

PR: https://git.openjdk.org/jdk/pull/25117