remove very likely unused pytest_plugin_unregister hook (pytest itself and all plugins i know don't use it)

_pytest/core.py

@@ -114,7 +114,6 @@ class PluginManager(object):
         if plugin is None:
             plugin = self.getplugin(name=name)
         self._plugins.remove(plugin)
-        self.hook.pytest_plugin_unregistered(plugin=plugin)
         for name, value in list(self._name2plugin.items()):
             if value == plugin:
                 del self._name2plugin[name]