Always do remembered set verification during G1 Full GC

Currently by default we do not verify the remembered sets during full gc as guarded by the G1VerifyRSetsDuringFullGC flag.

This has apparently a way to limit time spent in verification during full gc, introduced sometime in the first versions of the g1 collector.

Nowadays we can limit verification much more flexibly using e.g. GCVerifyType. Further, since full gc is rare, this particular additional verification does not matter.