kernel: Remove gSchedulerLock

* Thread::scheduler_lock protects thread state, priority, etc.
 * sThreadCreationLock protects thread creation and removal and list of
   threads in team.
 * Team::signal_lock and Team::time_lock protect list of threads in team
   as well.
 * Scheduler uses its own internal locking.

headers/private/kernel/thread.h

@@ -69,6 +69,9 @@ public:
 using BKernel::ThreadCreationAttributes;
 
 
+extern spinlock gThreadCreationLock;
+
+
 #ifdef __cplusplus
 extern "C" {
 #endif