Remove broken/unused PytestPluginManager.addhooks

The function has been deprecated for ages and
the PLUGIN_MANAGER_ADDHOOKS constant doesn't even exist since 3.0.

Because the function is clearly broken, this change doesn't even
require a CHANGELOG.

src/_pytest/config/__init__.py

@@ -23,7 +23,6 @@
 from .exceptions import UsageError
 from .findpaths import determine_setup
 from .findpaths import exists
-from _pytest import deprecated
 from _pytest._code import ExceptionInfo
 from _pytest._code import filter_traceback
 from _pytest.outcomes import fail
@@ -242,16 +241,6 @@ def __init__(self):
         # Used to know when we are importing conftests after the pytest_configure stage
         self._configured = False
 
-    def addhooks(self, module_or_class):
-        """
-        .. deprecated:: 2.8
-
-        Use :py:meth:`pluggy.PluginManager.add_hookspecs <PluginManager.add_hookspecs>`
-        instead.
-        """
-        warnings.warn(deprecated.PLUGIN_MANAGER_ADDHOOKS, stacklevel=2)
-        return self.add_hookspecs(module_or_class)
-
     def parse_hookimpl_opts(self, plugin, name):
         # pytest hooks are always prefixed with pytest_
         # so we avoid accessing possibly non-readable attributes