remove suprerfluous thread_yield declaration

cpu/msp430-common/cpu.c

@@ -14,6 +14,7 @@ See the file LICENSE in the top level directory for more details.
 #include "kernel.h"
 #include "kernel_internal.h"
 #include "sched.h"
+#include "thread.h"
 
 volatile int __inISR = 0;
 

cpu/msp430-common/include/cpu.h

@@ -125,9 +125,6 @@ inline void dINT(void)
     dint();
 }
 
-void thread_yield(void);
-
-
 int inISR(void);
 
 void msp430_cpu_init(void);