[lworld] RFR: 8348904: [lworld] Remove concept of default value object and zeroInstance

[Roger Riggs]

Remove support for the default value object and zeroInstance, no longer included in the spec.
In related tests, test cases were removed if no longer applicable or replaced by a hand created zero/null object.

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Commit messages:
 - 8348904: [lworld] Remove concept of default value object and zeroInstance

Changes: https://git.openjdk.org/valhalla/pull/1346/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1346&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348904
  Stats: 99 lines in 13 files changed: 4 ins; 60 del; 35 mod
  Patch: https://git.openjdk.org/valhalla/pull/1346.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1346/head:pull/1346

PR: https://git.openjdk.org/valhalla/pull/1346