Remove HTMLParserScriptRunner's prefinalizer
We don't have to Dispose() PendingScripts, because:
- If HTMLParserScriptRunner is watching for load of a PendingScript, then
the PendingScript has a Member<HTMLParserScriptRunner> of |this|,
and thus at prefinalization the PendingScript is also to be garbage
collected. Dispose() is called by PendingScript's prefinalizer.
- Otherwise, what matters in PendingScript::Dispose() is
ScriptStreamer::Cancel() in ClassicPendingScript::DisposeInternal().
This CL might defer that ScriptStreamer::Cancel() to when
ClassicPendingScript is prefinalized, but this shouldn't cause anything