Refactor SsoHandler.get_mxid_from_sso (#8900)

* Factor out _call_attribute_mapper and _register_mapped_user

This is mostly an attempt to simplify `get_mxid_from_sso`.

* Move mapping_lock down into SsoHandler.

synapse/handlers/saml_handler.py

@@ -34,7 +34,6 @@ from synapse.types import (
     map_username_to_mxid_localpart,
     mxid_localpart_allowed_characters,
 )
-from synapse.util.async_helpers import Linearizer
 from synapse.util.iterutils import chunk_seq
 
 if TYPE_CHECKING:
@@ -81,9 +80,6 @@ class SamlHandler(BaseHandler):
         # a map from saml session id to Saml2SessionData object
         self._outstanding_requests_dict = {}  # type: Dict[str, Saml2SessionData]
 
-        # a lock on the mappings
-        self._mapping_lock = Linearizer(name="saml_mapping", clock=self.clock)
-
         self._sso_handler = hs.get_sso_handler()
 
     def handle_redirect_request(
@@ -299,15 +295,14 @@ class SamlHandler(BaseHandler):
 
             return None
 
-        with (await self._mapping_lock.queue(self._auth_provider_id)):
-            return await self._sso_handler.get_mxid_from_sso(
-                self._auth_provider_id,
-                remote_user_id,
-                user_agent,
-                ip_address,
-                saml_response_to_remapped_user_attributes,
-                grandfather_existing_users,
-            )
+        return await self._sso_handler.get_mxid_from_sso(
+            self._auth_provider_id,
+            remote_user_id,
+            user_agent,
+            ip_address,
+            saml_response_to_remapped_user_attributes,
+            grandfather_existing_users,
+        )
 
     def _remote_id_from_saml_response(
         self,