[sankaku] remove login support

The old login method for 'https://chan.sankakucomplex.com/user/login'
and the cookies it produces have no effect on the results from
'beta.sankakucomplex.com'.

README.rst

@@ -213,9 +213,18 @@ Some extractors require you to provide valid login credentials in the form of
 a username & password pair. This is necessary for
 ``pixiv``, ``nijie``, and ``seiga``
 and optional for
-``aryion``, ``danbooru``, ``e621``, ``exhentai``, ``idolcomplex``, ``inkbunny``,
-``instagram``, ``luscious``, ``pinterest``,  ``sankaku``, ``subscribestar``,
-``tsumino``, and ``twitter``.
+``aryion``,
+``danbooru``,
+``e621``,
+``exhentai``,
+``idolcomplex``,
+``inkbunny``,
+``instagram``,
+``luscious``,
+``pinterest``,
+``subscribestar``,
+``tsumino``,
+and ``twitter``.
 
 You can set the necessary information in your configuration file
 (cf. gallery-dl.conf_)