RFR: 8287771: Remove useless G1 After GC summary refinement and sampling thread times [v3]

[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.

tqxia has updated the pull request incrementally with one additional commit since the last revision:

  Update flag name to show_thread_times and add comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9106/files
  - new: https://git.openjdk.org/jdk/pull/9106/files/a2c7dde2..c09adf08

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9106&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9106&range=01-02

  Stats: 12 lines in 5 files changed: 4 ins; 0 del; 8 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