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.

src/message.rs

@@ -1319,7 +1319,7 @@ impl Message {
     }
 
     /// Force the message to be sent in plain text.
-    pub fn force_plaintext(&mut self) {
+    pub(crate) fn force_plaintext(&mut self) {
         self.param.set_int(Param::ForcePlaintext, 1);
     }