Remove unused headers.

Push Closure into only use in thread_pool.h.
Remove StringFile and NullFile that existed purely for a test.

Change-Id: I329892d5e1f5f017caeb3d60600f803a74114fc1
diff --git a/runtime/thread_pool.h b/runtime/thread_pool.h
index d6330c8..8c08067 100644
--- a/runtime/thread_pool.h
+++ b/runtime/thread_pool.h
@@ -22,13 +22,18 @@
 
 #include "barrier.h"
 #include "base/mutex.h"
-#include "closure.h"
 #include "mem_map.h"
 
 namespace art {
 
 class ThreadPool;
 
+class Closure {
+ public:
+  virtual ~Closure() { }
+  virtual void Run(Thread* self) = 0;
+};
+
 class Task : public Closure {
  public:
   // Called when references reaches 0.