Remove ModRefBarrierSetC2::pre_barrier(), whose only implementation after JDK-8334060 is empty [1]; and remove the following arguments of ModRefBarrierSetC2::post_barrier(), which are not used anymore: Node* ctl, Node* store, uint adr_idx, and BasicType bt.

[1] https://github.com/openjdk/jdk/blob/cc198972022c94199d698461e2ac42afc0058fd7/src/hotspot/share/gc/shared/c2/modRefBarrierSetC2.hpp#L34-L43

Thanks to [~thartmann] for reporting.