Revert "Remove any NULL characters from displaynames to avoid breaking database txn"

This reverts commit a15e12c which has now been
fixed in upstream.

Author: Fizzadar

synapse/handlers/user_directory.py

@@ -464,10 +464,6 @@ async def _handle_possible_remote_profile_change(
 
         prev_name = prev_event.content.get("displayname")
         new_name = event.content.get("displayname")
-
-        # Replace any NULL characters in the name as these cannot be stored in the database
-        new_name = new_name.replace("\x00", "\uFFFD")
-
         # If the new name is an unexpected form, do not update the directory.
         if not isinstance(new_name, str):
             new_name = prev_name