Remove runtime code not needed with CMS removed

in classLoaderData.hpp, these (and associated code) were added for CMS:

  // Remembered sets support for the oops in the class loader data.
  bool _modified_oops; // Card Table Equivalent (YC/CMS support) (might be used for ParallelGC tho)
  bool _accumulated_modified_oops; // Mod Union Equivalent (CMS support)

in classLoaderDataGraph.hpp, these are for CMS:

  // CMS support.
  static ClassLoaderData* _saved_head;
  static ClassLoaderData* _saved_unloading;
  static bool _should_purge;

in safepoint.cpp, this is for CMS. I don't think any other GCs delay purging ClassLoaderData until the next safepoint:

    if (_subtasks.try_claim_task(SafepointSynchronize::SAFEPOINT_CLEANUP_CLD_PURGE)) {
      if (ClassLoaderDataGraph::should_purge_and_reset()) {
        // CMS delays purging the CLDG until the beginning of the next safepoint and to
        // make sure concurrent sweep is done
        const char* name = "purging class loader data graph";
        EventSafepointCleanupTask event;
        TraceTime timer(name, TRACETIME_LOG(Info, safepoint, cleanup));
        ClassLoaderDataGraph::purge();

        post_safepoint_cleanup_task_event(event, safepoint_id, name);
      }
    }