Remove check_ct_logs_at_safepoint()

The function G1CollectedHeap::check_ct_logs_at_safepoint() has no callers. It was added as part of the initial open source merge of G1 into the hotspot code base (https://bugs.openjdk.java.net/browse/JDK-6711316), and had no callers then either. Its purpose is to perform a consistency check between the card table and the dirty card logs. If something along those lines is needed for some future debugging task, it's easy enough to write a bespoke test when that time comes.

Since this function isn't needed, it should be removed. Removal of that function also renders the helper class ClearLoggedCardTableEntryClosure unused, allowing it to be removed as well.