Revert "Fix existing v2 identity server calls (MSC2140) (#6013)"

This has now been merged into develop (3505ffcda) so we don't need this
cherry-picked commit.

This reverts commit e0eef47315.

changelog.d/6013.misc

@@ -1 +0,0 @@
-Compatibility with v2 Identity Service APIs other than /lookup.

synapse/handlers/identity.py

@@ -74,25 +74,6 @@ class IdentityHandler(BaseHandler):
         id_access_token = creds.get("id_access_token")
         return client_secret, id_server, id_access_token
 
-    def create_id_access_token_header(self, id_access_token):
-        """Create an Authorization header for passing to SimpleHttpClient as the header value
-        of an HTTP request.
-
-        Args:
-            id_access_token (str): An identity server access token.
-
-        Returns:
-            list[str]: The ascii-encoded bearer token encased in a list.
-        """
-        # Prefix with Bearer
-        bearer_token = "Bearer %s" % id_access_token
-
-        # Encode headers to standard ascii
-        bearer_token.encode("ascii")
-
-        # Return as a list as that's how SimpleHttpClient takes header values
-        return [bearer_token]
-
     @defer.inlineCallbacks
     def threepid_from_creds(self, id_server, creds):
         """
@@ -168,20 +149,15 @@ class IdentityHandler(BaseHandler):
             use_v2 = False
 
         # Decide which API endpoint URLs to use
-        headers = {}
         bind_data = {"sid": creds["sid"], "client_secret": client_secret, "mxid": mxid}
         if use_v2:
             bind_url = "https://%s/_matrix/identity/v2/3pid/bind" % (id_server,)
-            headers["Authorization"] = self.create_id_access_token_header(
-                id_access_token
-            )
+            bind_data["id_access_token"] = id_access_token
         else:
             bind_url = "https://%s/_matrix/identity/api/v1/3pid/bind" % (id_server,)
 
         try:
-            data = yield self.http_client.post_json_get_json(
-                bind_url, bind_data, headers=headers
-            )
+            data = yield self.http_client.post_json_get_json(bind_url, bind_data)
             logger.debug("bound threepid %r to %s", creds, mxid)
 
             # Remember where we bound the threepid