ART: Remove old object_callbacks.h includes

Remove unused includes.

Test: m
Change-Id: I70c227d32c6900904e186b975d1f2131b718d93d
diff --git a/runtime/java_vm_ext.h b/runtime/java_vm_ext.h
index 7374920..50aabdc 100644
--- a/runtime/java_vm_ext.h
+++ b/runtime/java_vm_ext.h
@@ -32,6 +32,7 @@
 }  // namespace mirror
 
 class ArtMethod;
+class IsMarkedVisitor;
 class Libraries;
 class ParsedOptions;
 class Runtime;