RFR: 8276121: G1: Remove unused and uninitialized _g1h in g1SATBMarkQueueSet.hpp

[Leo Korinth]

Code seems to compile without the field.

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Commit messages:
 - 8276121: G1: Remove unused and uninitialized _g1h in g1SATBMarkQueueSet.hpp

Changes: https://git.openjdk.java.net/jdk/pull/6171/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6171&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276121
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6171.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6171/head:pull/6171

PR: https://git.openjdk.java.net/jdk/pull/6171