Remove unnecessary `explicit` qualifiers on constructors.

Change-Id: Id12e392ad50f66a6e2251a68662b7959315dc567
diff --git a/runtime/base/arena_allocator.h b/runtime/base/arena_allocator.h
index c4b36ee..05c66f0 100644
--- a/runtime/base/arena_allocator.h
+++ b/runtime/base/arena_allocator.h
@@ -170,7 +170,7 @@
 
 class MemMapArena FINAL : public Arena {
  public:
-  explicit MemMapArena(size_t size, bool low_4gb);
+  MemMapArena(size_t size, bool low_4gb);
   virtual ~MemMapArena();
   void Release() OVERRIDE;