Fix scheduled plugin unloading

When unloading a plugin no plugin is allowed to execute code. There's
been some flawed logic in the scheduling, leading it to bail out if the
plugin was marked for scheduled unloading..

Also change the "error" message of `sm plugins unload` to something
nice.

core/logic/PluginSys.h

@@ -462,7 +462,6 @@ private:
 	bool RequireExtensions(CPlugin *pPlugin);
 	bool FindOrRequirePluginDeps(CPlugin *pPlugin);
 
-	bool ScheduleUnload(CPlugin *plugin);
 	void UnloadPluginImpl(CPlugin *plugin);
 
 	void Purge(CPlugin *plugin);