RFR: 8333713: C2 SuperWord: cleanup in vectornode.cpp/hpp

[Emanuel Peter]

Removed dead code and renamed `superword` -> `auto_vectorization`, just like I had done in [JDK-8324750](https://bugs.openjdk.org/browse/JDK-8324750).

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Commit messages:
 - 8333713

Changes: https://git.openjdk.org/jdk/pull/19575/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19575&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333713
  Stats: 16 lines in 2 files changed: 0 ins; 10 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/19575.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19575/head:pull/19575

PR: https://git.openjdk.org/jdk/pull/19575