api!: remove dc_msg_force_plaintext

Message.force_plaintext() is still used in legacy SecureJoin steps
internally, so cannot be removed, but there is no need for public API.

python/src/deltachat/message.py

@@ -254,10 +254,6 @@ class Message:
         """Quote setter."""
         lib.dc_msg_set_quote(self._dc_msg, quoted_message._dc_msg)
 
-    def force_plaintext(self) -> None:
-        """Force the message to be sent in plain text."""
-        lib.dc_msg_force_plaintext(self._dc_msg)
-
     @property
     def error(self) -> Optional[str]:
         """Error message."""