Remove unused ZODB.tests.warnhook

The last usage of WarningsHook was removed in 2011 in f1b04dd8 (Lots of
code cleanups.). And when we need to capture warnings there is
`warnings.catch_warnings` that is available out of the box from stdlib
on both py2 and py3.

src/ZODB/tests/warnhook.py

-##############################################################################
-#
-# Copyright (c) 2004 Zope Foundation and Contributors.
-# All Rights Reserved.
-#
-# This software is subject to the provisions of the Zope Public License,
-# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
-# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
-# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
-# FOR A PARTICULAR PURPOSE.
-#
-##############################################################################
-import warnings
-
-
-class WarningsHook(object):
-    """Hook to capture warnings generated by Python.
-
-    The function warnings.showwarning() is designed to be hooked by
-    application code, allowing the application to customize the way it
-    handles warnings.
-
-    This hook captures the unformatted warning information and stores
-    it in a list.  A test can inspect this list after the test is over.
-
-    Issues:
-
-    The warnings module has lots of delicate internal state.  If
-    a warning has been reported once, it won't be reported again.  It
-    may be necessary to extend this class with a mechanism for
-    modifying the internal state so that we can be guaranteed a
-    warning will be reported.
-
-    If Python is run with a warnings filter, e.g. python -Werror,
-    then a test that is trying to inspect a particular warning will
-    fail.  Perhaps this class can be extended to install more-specific
-    filters the test to work anyway.
-    """
-
-    def __init__(self):
-        self.original = None
-        self.warnings = []
-
-    def install(self):
-        self.original = warnings.showwarning
-        warnings.showwarning = self.showwarning
-
-    def uninstall(self):
-        assert self.original is not None
-        warnings.showwarning = self.original
-        self.original = None
-
-    def showwarning(self, message, category, filename, lineno):
-        self.warnings.append((str(message), category, filename, lineno))
-
-    def clear(self):
-        self.warnings = []