G1: Remove PreservedMarks::init_forwarded_mark

JDK-8152312 introduced PreservedMarks::init_forwarded_mark for `RemoveForwardedPointerClosure`. Now that `RemoveForwardedPointerClosure` is gone (in JDK-8281035), `init_forwarded_mark` can be dropped as well.

The sole caller of `init_forwarded_mark` is in `RemoveSelfForwardPtrObjClosure`; using `init_mark()` is more desirable because in that context we don't work with "preserved marks".