Simplify oops_on_card_seq_iterate_careful

HeapRegion::oops_on_card_seq_iterate_careful has an interface and implementation that try to be (somewhat) general and support multiple caller contexts. However, there is actually only one caller, G1RemSet::refine_card, which does not use or need that generality.

This unused generality makes the code harder to understand. In particular, it makes the analysis needed to address JDK-8160369, due to the need to consider effectively unused code paths.