remove test for page state saved for disabled plugin

My change did cause this state to be retained. I hope this is not a
problem.

Afaik, plugins test if they were disabled before by looking at the toplevel
plugin state, not the per-page plugin state. So the only remaining problem
might be

a) A plugin is disabled but its state keeps being saved. Which is not
   ideal, perhaps, but the large speedup of my optimisation seems worth it.
b) A plugin might have been enabled, be disabled, and get re-enabled, and
   see old state from before. I don't see how this would be different from
   the plugin seeing any other old state, though, so hopefully no breakage.

My optmisation looks a little more risky, but I still hope I can keep it.

t/index.t

@@ -38,8 +38,6 @@ $depends{"bar"}={"foo*" => 1};
 $depends{"bar.png"}={};
 $pagestate{"bar"}{meta}{title}="a page about bar";
 $pagestate{"bar"}{meta}{moo}="mooooo";
-# only loaded plugins save state, so this should not be saved out
-$pagestate{"bar"}{nosuchplugin}{moo}="mooooo";
 
 ok(saveindex(), "save index");
 ok(-s "$config{wikistatedir}/indexdb", "index file created");