Remove MethodHelper::HasSameNameAndSignature.

Move sole use to a static method with art_method.cc.

Change-Id: I2e7994cc1c31b5ca74df5d7be5538003d4ed0150
diff --git a/runtime/thread.h b/runtime/thread.h
index b69d2f4..5b3e746 100644
--- a/runtime/thread.h
+++ b/runtime/thread.h
@@ -1189,7 +1189,7 @@
 
  private:
   Thread* const self_;
-  const char* old_cause_;
+  const char* const old_cause_;
 };
 
 std::ostream& operator<<(std::ostream& os, const Thread& thread);