[code-reflection] RFR: Replace methods "create" in Op subclasses by constructors

[Mourad Abbay]

By using flexible constructor bodies feature, we get rid of methods "create" in Op subclasses and we replace it with constructors. We also remove unused constructors and methods in Op subclasses.

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Commit messages:
 - Remove unused constructors and methods
 - Replace methods that create Ops from ExternalizedOp, by constructors

Changes: https://git.openjdk.org/babylon/pull/584/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=584&range=00
  Stats: 95 lines in 2 files changed: 4 ins; 32 del; 59 mod
  Patch: https://git.openjdk.org/babylon/pull/584.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/584/head:pull/584

PR: https://git.openjdk.org/babylon/pull/584