Delete ClassHelper and fix compaction bug in GetDirectInterface

Cleanup helps to prevent compaction bugs. Fixed a fairly serious
compaction error caused by calling ClassHelper::GetDirectInterface
without handling the case where it causes thread suspension due to
ResolveType.

Bug: 8981901

Change-Id: I82b3bb6dd48d21eb6ece7aae0733c4a23c2bc408
diff --git a/runtime/class_linker-inl.h b/runtime/class_linker-inl.h
index ce634e0..ac86014 100644
--- a/runtime/class_linker-inl.h
+++ b/runtime/class_linker-inl.h
@@ -49,7 +49,7 @@
   }
   DCHECK(!element_class->IsPrimitiveVoid());
   std::string descriptor("[");
-  descriptor += ClassHelper(element_class).GetDescriptor();
+  descriptor += element_class->GetDescriptor();
   StackHandleScope<1> hs(Thread::Current());
   Handle<mirror::ClassLoader> class_loader(hs.NewHandle(element_class->GetClassLoader()));
   mirror::Class* array_class = FindClass(self, descriptor.c_str(), class_loader);