RFR: 8261657: [PPC64] Cleanup StoreCM nodes after CMS removal

[Martin Doerr]

We only need one StoreCM node after CMS removal. CMS StoreStore barriers were already removed at other places.

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Commit messages:
 - 8261657: [PPC64] Cleanup StoreCM nodes after CMS removal

Changes: https://git.openjdk.java.net/jdk/pull/2554/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2554&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261657
  Stats: 55 lines in 1 file changed: 0 ins; 52 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2554.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2554/head:pull/2554

PR: https://git.openjdk.java.net/jdk/pull/2554