RFR: 8258192: Obsolete the CriticalJNINatives flag

[Coleen Phillimore]

This change removes the disabled CriticalJNINatives code, and the flag now gives an obsolete message.
Tested with tier1 on cpus x64, aarch64, and builds on linux-x86-open,linux-s390x-open,linux-arm32-debug,linux-ppc64le-debug.

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Commit messages:
 - 8258192: Obsolete the CriticalJNINatives flag

Changes: https://git.openjdk.java.net/jdk/pull/6343/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6343&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258192
  Stats: 1790 lines in 24 files changed: 0 ins; 1616 del; 174 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6343.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6343/head:pull/6343

PR: https://git.openjdk.java.net/jdk/pull/6343