[code-reflection] RFR: OnnxProtoBuilder simplifications

[Adam Sotona]

OnnxProtoBuilder simplifications

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Commit messages:
 - more simplification
 - OnnxProtoBuilder simplifications

Changes: https://git.openjdk.org/babylon/pull/324/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=324&range=00
  Stats: 111 lines in 8 files changed: 33 ins; 44 del; 34 mod
  Patch: https://git.openjdk.org/babylon/pull/324.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/324/head:pull/324

PR: https://git.openjdk.org/babylon/pull/324