Kernel: Remove use of copy_ref() in favor of regular RefPtr copies.

This is obviously more readable. If we ever run into a situation where
ref count churn is actually causing trouble in the future, we can deal with
it then. For now, let's keep it simple. :^)

Kernel/Devices/MBRPartitionTable.h

@@ -31,7 +31,7 @@ class MBRPartitionTable {
     AK_MAKE_ETERNAL
 
 public:
-    MBRPartitionTable(NonnullRefPtr<DiskDevice>&& device);
+    MBRPartitionTable(NonnullRefPtr<DiskDevice>);
     ~MBRPartitionTable();
 
     bool initialize();