Rename G1RemSet::*oops_into_collection_set_do methods

In G1RemSet code there are two methods to set up and finalize remembered set scanning called

prepare_for_oops_into_collection_set_do
cleanup_after_oops_into_collection_set_do

which refer to an oops_into_collection_set_do method that has been renamed to scan_rem_set for a long time. Fix up the method names.