remove python 2 reminiscence in cookies

scrapy/http/cookies.py

@@ -186,9 +186,6 @@ class WrappedResponse:
     def info(self):
         return self
 
-    # python3 cookiejars calls get_all
     def get_all(self, name, default=None):
         return [to_unicode(v, errors='replace')
                 for v in self.response.headers.getlist(name)]
-    # python2 cookiejars calls getheaders
-    getheaders = get_all

tests/test_http_cookies.py

@@ -64,9 +64,6 @@ class WrappedResponseTest(TestCase):
     def test_info(self):
         self.assertIs(self.wrapped.info(), self.wrapped)
 
-    def test_getheaders(self):
-        self.assertEqual(self.wrapped.getheaders('content-type'), ['text/html'])
-
     def test_get_all(self):
         # get_all result must be native string
         self.assertEqual(self.wrapped.get_all('content-type'), ['text/html'])