api: deprecate webxdc descr parameter (#6255)

this PR removes most usages of the `descr` parameter.

- to avoid noise in different branches etc. (as annoying on similar, at
a first glance simple changes), i left the external API stable

- also, the effort to do a database migration seems to be over the top,
so the column is left and set to empty strings on future updates - maybe
we can recycle the column at some point ;)

closes #6245

src/download.rs

@@ -440,7 +440,7 @@ mod tests {
         let _sent1 = alice.send_msg(chat_id, &mut instance).await;
 
         alice
-            .send_webxdc_status_update(instance.id, r#"{"payload":7}"#, "d")
+            .send_webxdc_status_update(instance.id, r#"{"payload":7}"#)
             .await?;
         alice.flush_status_updates().await?;
         let sent2 = alice.pop_sent_msg().await;