[code-reflection] RFR: Remove CodeElement.transform

[Paul Sandoz]

Remove the `CodeElement.transform` method and the helper methods. Traversal of elements is performed using the `CodeElement.elements` method that returns a stream of code elements.

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Commit messages:
 - Remove comments.
 - Merge remote-tracking branch 'upstream/code-reflection' into rm-ce-transform
 - Remove CodeElement.transform method.

Changes: https://git.openjdk.org/babylon/pull/737/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=737&range=00
  Stats: 235 lines in 20 files changed: 47 ins; 91 del; 97 mod
  Patch: https://git.openjdk.org/babylon/pull/737.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/737/head:pull/737

PR: https://git.openjdk.org/babylon/pull/737