G1RemSetSummary::_rs_threads_vtimes is not initialized to zero in the constructor "G1RemSetSummary(G1RemSet* rem_set)", this leads to invalid values in GC logs for "Concurrent refinement threads times (s)".

Reproducible with:
$ java -XX:+UnlockDiagnosticVMOptions -XX:G1SummarizeRSetStatsPeriod=1 -Xlog:gc+remset*=trace -XX:ParallelGCThreads=8 -version
(It is more obvious in a fastdebug build.)

In addition, after JDK-8231153, the field G1RemSetSummary::_rem_set is never used. Removing this field and merging the two constructors of G1RemSetSummary will fix the bug and clean up the code.