RFR: 8288984: Simplification in Shutdown.exit

[Ryan Ernst]

This is a followup to simplify Shutdown.exit after the removal of
finalizers (https://bugs.openjdk.org/browse/JDK-8198250). Once agreement
on the approach has been reached in this PR, a CSR will be filed to
propose the spec change to Runtime.exit.

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Commit messages:
 - 8288984: Simplification in Shutdown.exit

Changes: https://git.openjdk.org/jdk/pull/9351/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9351&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288984
  Stats: 10 lines in 2 files changed: 0 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9351.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9351/head:pull/9351

PR: https://git.openjdk.org/jdk/pull/9351