kernel: Remove possibility to yield to all threads

Kernel support for yielding to all (including lower priority) threads
has been removed. POSIX sched_yield() remains unchanged.

If a thread really needs to yield to everyone it can reduce its priority
to the lowest possible and then yield (it will then need to manually
return to its prvious priority upon continuing).

headers/private/kernel/thread.h

@@ -86,7 +86,7 @@ void thread_reset_for_exec(void);
 
 status_t thread_init(struct kernel_args *args);
 status_t thread_preboot_init_percpu(struct kernel_args *args, int32 cpuNum);
-void thread_yield(bool force);
+void thread_yield(void);
 void thread_exit(void);
 
 int32 thread_max_threads(void);