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

CHANGELOG

@@ -11,6 +11,9 @@ known incompatibilities:
   unittest-style) will not be called if the corresponding setup method failed,
   see issue322 below.
 
+- the pytest_plugin_unregister hook wasn't ever properly called
+  and there is no known implementation of the hook - so it got removed.
+
 new features:
 
 - experimentally allow fixture functions to "yield" instead of "return"

_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]

_pytest/hookspec.py

@@ -236,10 +236,7 @@ pytest_doctest_prepare_content.firstresult = True
 # -------------------------------------------------------------------------
 
 def pytest_plugin_registered(plugin, manager):
-    """ a new py lib plugin got registered. """
-
-def pytest_plugin_unregistered(plugin):
-    """ a py lib plugin got unregistered. """
+    """ a new pytest plugin got registered. """
 
 def pytest_internalerror(excrepr, excinfo):
     """ called for internal errors. """