thread: Make the scheduler pointer a regular pointer

Conceptually, it doesn't make sense for a thread to be able to persist
the lifetime of a scheduler. A scheduler should be taking care of the
threads; the threads should not be taking care of the scheduler.

If the threads outlive the scheduler (or we simply don't actually
terminate/shutdown the threads), then it should be considered a bug
that we need to fix.

Attributing this to balika011, as they opened #1317 to attempt to fix
this in a similar way, but my refactoring of the kernel code caused
quite a few conflicts.

src/core/hle/kernel/thread.h

@@ -419,7 +419,7 @@ private:
     /// available. In case of a timeout, the object will be nullptr.
     WakeupCallback wakeup_callback;
 
-    std::shared_ptr<Scheduler> scheduler;
+    Scheduler* scheduler = nullptr;
 
     u32 ideal_core{0xFFFFFFFF};
     u64 affinity_mask{0x1};