Rip out half-implemented m.login.saml2 support (#4265)

* Rip out half-implemented m.login.saml2 support

This was implemented in an odd way that left most of the work to the client, in
a way that I really didn't understand. It's going to be a pain to maintain, so
let's start by ripping it out.

* drop undocumented dependency on dateutil

It turns out we were relying on dateutil being pulled in transitively by
pysaml2. There's no need for that bloat.

synapse/config/saml2.py

@@ -1,55 +0,0 @@
-# -*- coding: utf-8 -*-
-# Copyright 2015 Ericsson
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-from ._base import Config
-
-
-class SAML2Config(Config):
-    """SAML2 Configuration
-    Synapse uses pysaml2 libraries for providing SAML2 support
-
-    config_path:      Path to the sp_conf.py configuration file
-    idp_redirect_url: Identity provider URL which will redirect
-                      the user back to /login/saml2 with proper info.
-
-    sp_conf.py file is something like:
-    https://github.com/rohe/pysaml2/blob/master/example/sp-repoze/sp_conf.py.example
-
-    More information: https://pythonhosted.org/pysaml2/howto/config.html
-    """
-
-    def read_config(self, config):
-        saml2_config = config.get("saml2_config", None)
-        if saml2_config:
-            self.saml2_enabled = saml2_config.get("enabled", True)
-            self.saml2_config_path = saml2_config["config_path"]
-            self.saml2_idp_redirect_url = saml2_config["idp_redirect_url"]
-        else:
-            self.saml2_enabled = False
-            self.saml2_config_path = None
-            self.saml2_idp_redirect_url = None
-
-    def default_config(self, config_dir_path, server_name, **kwargs):
-        return """
-        # Enable SAML2 for registration and login. Uses pysaml2
-        # config_path:      Path to the sp_conf.py configuration file
-        # idp_redirect_url: Identity provider URL which will redirect
-        #                   the user back to /login/saml2 with proper info.
-        # See pysaml2 docs for format of config.
-        #saml2_config:
-        #   enabled: true
-        #   config_path: "%s/sp_conf.py"
-        #   idp_redirect_url: "http://%s/idp"
-        """ % (config_dir_path, server_name)