Kernel: Don't register thread as custom data for WaitQueueBlocker

When adding a WaitQueueBlocker to a WaitQueue, it stored the blocked
thread in the registration's custom "void* data" slot.
This was only used to print the Thread* in some debug logging.

Now that Blocker always knows its origin Thread, we can simply add
a Blocker::thread() accessor and then get the blocked Thread& from
there. No need to register custom data.

Kernel/Thread.h

@@ -301,6 +301,8 @@ public:
         virtual const BlockTimeout& override_timeout(const BlockTimeout& timeout) { return timeout; }
         virtual bool can_be_interrupted() const { return true; }
 
+        Thread& thread() { return m_thread; }
+
         enum class UnblockImmediatelyReason {
             UnblockConditionAlreadyMet,
             TimeoutInThePast,