feat: Remove the largely-unused ability to send multiple reactions to one message (#8131)

After talking with r10s:

For spring cleaning, remove the largely-unused things that can be done a
bit.

Most private messengers (WhatsApp/Signal/...) do not have this feature,
and we do not want to become Matrix where every client has different,
partly-incompatible features.

deltachat-rpc-client/src/deltachat_rpc_client/message.py

@@ -25,7 +25,14 @@ class Message:
         return self.account._rpc
 
     def send_reaction(self, *reaction: str) -> "Message":
-        """Send a reaction to this message."""
+        """
+        Sends a reaction to message.
+
+        A reaction is a string that represents an emoji.
+        You can call this function again to change the emoji;
+        the last sent reaction overrides all previously sent reactions.
+        It is possible to remove the reaction by sending an empty string.
+        """
         msg_id = self._rpc.send_reaction(self.account.id, self.id, reaction)
         return Message(self.account, msg_id)