DVDInterface: Remove GetVolume

For thread safety reasons, the currently inserted volume must
only be accessed by the DVD thread (or by the CPU thread if it
calls DVDThread::WaitUntilIdle() first). After this commit,
only DVDThread.cpp can access the volume, which prevents code in
other files from accessing the volume in a non-threadsafe way.

Source/Core/Core/State.cpp

@@ -71,7 +71,7 @@ static Common::Event g_compressAndDumpStateSyncEvent;
 static std::thread g_save_thread;
 
 // Don't forget to increase this after doing changes on the savestate system
-static const u32 STATE_VERSION = 84;  // Last changed in PR 5354
+static const u32 STATE_VERSION = 85;  // Last changed in PR 4241
 
 // Maps savestate versions to Dolphin versions.
 // Versions after 42 don't need to be added to this list,