[base] Drop obsolete Thread::YieldCPU.

src/base/platform/platform.h

Index: src/base/platform/platform.h
diff --git a/src/base/platform/platform.h b/src/base/platform/platform.h
index 2d837baff61078fedd74626491dc28db8d3bf930..f54eef3c44f10da76a21ab2f04dbc4bea5feceec 100644
--- a/src/base/platform/platform.h
+++ b/src/base/platform/platform.h
@@ -446,10 +446,6 @@ class Thread {
   }
 #endif
 
-  // A hint to the scheduler to let another thread run.
-  static void YieldCPU();
-
-
   // The thread name length is limited to 16 based on Linux's implementation of
   // prctl().
   static const int kMaxThreadNameLength = 16;