Kernel+LibC: Remove sys$donate()

This was an old SerenityOS-specific syscall for donating the remainder
of the calling thread's time-slice to another thread within the same
process.

Now that Threading::Lock uses a pthread_mutex_t internally, we no
longer need this syscall, which allows us to get rid of a surprising
amount of unnecessary scheduler logic. :^)

Kernel/Thread.h

@@ -1337,7 +1337,6 @@ private:
     bool m_is_profiling_suppressed { false };
 
     void yield_without_holding_big_lock();
-    void donate_without_holding_big_lock(RefPtr<Thread>&, const char*);
     void yield_while_not_holding_big_lock();
     void drop_thread_count(bool);
 };