Remove declaration of undefined method.

Change-Id: I8b7bec33e4bc9082d6bbf005475c67afbe6d2913
diff --git a/runtime/gc/space/space.h b/runtime/gc/space/space.h
index ca01c55..b5837d0 100644
--- a/runtime/gc/space/space.h
+++ b/runtime/gc/space/space.h
@@ -112,7 +112,6 @@
   bool IsZygoteSpace() const {
     return GetType() == kSpaceTypeZygoteSpace;
   }
-  DlMallocSpace* AsZygoteSpace();
 
   // Does this space hold large objects and implement the large object space abstraction?
   bool IsLargeObjectSpace() const {