RFR: 8287771: Remove useless G1 After GC summary refinement and sampling thread times

[tqxia]

In the "After GC summary" printed by gc+remset=trace and G1RemsetStatsSummaryPeriod > 1 we print refinement and sampling thread times. However during GC neither thread types are active, so it always prints zeros.

Remove the printing of these unnecessary values in the after gc summary.

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Commit messages:
 - 8287771: Remove useless G1 After GC summary refinement and sampling thread times

Changes: https://git.openjdk.org/jdk/pull/9106/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9106&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287771
  Stats: 20 lines in 5 files changed: 5 ins; 3 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/9106.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9106/head:pull/9106

PR: https://git.openjdk.org/jdk/pull/9106