Stop reference-counting the ScriptTask.

The ScriptTask is owned by the proc in ScriptTaskFactory::create.

We started using a managed box for what would eventually become ScriptTask in
d21d27e08633b6a7f2b774a28c65c81d1eedbeaf.
auto merge of #3570 : Ms2ger/servo/rc-scripttask, r=metajack

The ScriptTask is owned by the proc in ScriptTaskFactory::create.

We started using a managed box for what would eventually become ScriptTask in
d21d27e08633b6a7f2b774a28c65c81d1eedbeaf.