Remove remaining is_guest argument uses from get_room_data calls (#8181)

#8174 removed the `is_guest` parameter from `get_room_data`, at the same time that #8157 was merged using it, colliding together to break unit tests on develop.

This PR removes the `is_guest` parameter from the call in the broken test.

Uses the same changelog as #8174.

changelog.d/8181.misc

@@ -0,0 +1 @@
+Remove unused `is_guest` parameter from, and add safeguard to, `MessageHandler.get_room_data`.

tests/rest/client/test_shadow_banned.py

@@ -271,11 +271,7 @@ def test_displayname(self):
         message_handler = self.hs.get_message_handler()
         event = self.get_success(
             message_handler.get_room_data(
-                self.banned_user_id,
-                room_id,
-                "m.room.member",
-                self.banned_user_id,
-                False,
+                self.banned_user_id, room_id, "m.room.member", self.banned_user_id,
             )
         )
         self.assertEqual(
@@ -308,11 +304,7 @@ def test_room_displayname(self):
         message_handler = self.hs.get_message_handler()
         event = self.get_success(
             message_handler.get_room_data(
-                self.banned_user_id,
-                room_id,
-                "m.room.member",
-                self.banned_user_id,
-                False,
+                self.banned_user_id, room_id, "m.room.member", self.banned_user_id,
             )
         )
         self.assertEqual(