RFR: 8261803: Remove unused TaskTerminator in g1 full gc ref proc executor

[Leo Korinth]

8261803: Remove unused TaskTerminator in g1 full gc ref proc executor

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Commit messages:
 - 8261803: Remove unused TaskTerminator in g1 full gc ref proc executor

Changes: https://git.openjdk.java.net/jdk/pull/2629/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2629&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261803
  Stats: 5 lines in 2 files changed: 0 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2629.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2629/head:pull/2629

PR: https://git.openjdk.java.net/jdk/pull/2629