G1: Replace G1PLAB with PLAB

Before the changes for 6888336, the G1PLAB class which is used for GCLABs in G1 had several extra fields, including a local bitmap used to explicitly mark objects during copies. Post the changes for 6888336, most of the extra fields are not needed any more. The only one that remains is the "retired" field. We should evaluate why it's there / whether it's still needed, remove G1PLAB, and use its superclass (PLAB) instead.