RFR: 8351165: Remove unused includes from vmStructs [v3]

[Coleen Phillimore]

> Please review this trivial change to remove some unneeded includes and type declarations from vmStructs.  The SystemDictionary type is unused because classes are looked up in the ClassLoaderDataGraph in the SA, and they haven't been in a single SystemDictionary for a very long time.
> Tested with SA tests locally.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Change the SystemDictionary section header comment too.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23897/files
  - new: https://git.openjdk.org/jdk/pull/23897/files/e33d5066..86443005

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23897&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23897&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23897.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23897/head:pull/23897

PR: https://git.openjdk.org/jdk/pull/23897