remove useless __yield alias for sched_yield

this is no longer used for anything, and reportedly clashed with a
builtin on certain compilers.

src/sched/sched_yield.c

 #include <sched.h>
 #include "syscall.h"
-#include "libc.h"
 
-int __yield()
+int sched_yield()
 {
 	return syscall(SYS_sched_yield);
 }
-
-weak_alias(__yield, sched_yield);