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-jsonrpc/src/api.rs

@@ -2377,12 +2377,12 @@ impl CommandApi {
         Ok(message_id.to_u32())
     }
 
-    /// Send a reaction to message.
+    /// Sends a reaction to message.
     ///
-    /// Reaction is a string of emojis separated by spaces. Reaction to a
-    /// single message can be sent multiple times. The last reaction
-    /// received overrides all previously received reactions. It is
-    /// possible to remove all reactions by sending an empty string.
+    /// 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.
     async fn send_reaction(
         &self,
         account_id: u32,