Remove unnecessary `explicit` qualifiers on constructors.

Change-Id: Id12e392ad50f66a6e2251a68662b7959315dc567
diff --git a/runtime/base/arena_containers.h b/runtime/base/arena_containers.h
index d6c4a54..a7aafdf 100644
--- a/runtime/base/arena_containers.h
+++ b/runtime/base/arena_containers.h
@@ -134,7 +134,7 @@
     typedef ArenaAllocatorAdapter<U> other;
   };
 
-  explicit ArenaAllocatorAdapter(ArenaAllocator* arena_allocator, ArenaAllocKind kind)
+  ArenaAllocatorAdapter(ArenaAllocator* arena_allocator, ArenaAllocKind kind)
       : ArenaAllocatorAdapterKind(kind),
         arena_allocator_(arena_allocator) {
   }