tests: Remove unnecessary maxDiff settings.

Now that ZulipTestCase does this, we don't need it in all these
individual test modules.

zerver/tests/test_bugdown.py

@@ -177,7 +177,6 @@ class BugdownTest(ZulipTestCase):
         # type: () -> None
         format_tests, linkify_tests = self.load_bugdown_tests()
 
-        self.maxDiff = None
         for name, test in six.iteritems(format_tests):
             converted = bugdown_convert(test['input'])
 
@@ -197,7 +196,6 @@ class BugdownTest(ZulipTestCase):
             return payload % ("<a href=\"%s\"%s title=\"%s\">%s</a>" % (href, target, href, url),)
 
         print("Running Bugdown Linkify tests")
-        self.maxDiff = None
         with mock.patch('zerver.lib.url_preview.preview.link_embed_data_from_cache', return_value=None):
             for inline_url, reference, url in linkify_tests:
                 try:

zerver/tests/test_events.py

@@ -344,9 +344,6 @@ class EventsRegisterTest(ZulipTestCase):
         # type: () -> None
         super(EventsRegisterTest, self).setUp()
         self.user_profile = self.example_user('hamlet')
-        self.maxDiff = None # type: Optional[int]
-    user_profile = get_user_profile_by_email('hamlet@zulip.com')
-    maxDiff = None
 
     def create_bot(self, email):
         # type: (str) -> UserProfile