RFR: 8253029: [PPC64] Remove obsolete Power6 code

[Martin Doerr]

The recent PPC64 platform files still contain code which is part of a Power6 optimization called Power6Scheduler.
However, this functionality was never completely contributed so it was never usable in the OpenJDK. We should remove it
since we're no longer optimizing for Power6.

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Commit messages:
 - Remove obsolete Power6 code

Changes: https://git.openjdk.java.net/jdk/pull/116/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=116&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253029
  Stats: 916 lines in 5 files changed: 0 ins; 899 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/116.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/116/head:pull/116

PR: https://git.openjdk.java.net/jdk/pull/116