RFR: 8335938: Review XxxBuilder.original and XxxModel.parent

[Chen Liang]

Remove unused `Class/Field/Method/CodeBuilder.original()`, and make `Field/Method/CodeModel.parent()` return present only if it's bound (i.e. not buffered transformed). See the CSR for details.

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Commit messages:
 - 8335938: Review XxxBuilder.original and XxxModel.parent

Changes: https://git.openjdk.org/jdk/pull/20177/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20177&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335938
  Stats: 92 lines in 12 files changed: 0 ins; 80 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/20177.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20177/head:pull/20177

PR: https://git.openjdk.org/jdk/pull/20177