Move removal of chat message tombstones to a separate function

DELETE operation may be slow compared to UPDATE. It is better to do in
a separate job.

src/chat.rs

@@ -2521,15 +2521,19 @@ pub fn delete_device_expired_messages(context: &Context) -> sql::Result<()> {
              WHERE timestamp < ?",
             params![threshold_timestamp],
         )?;
+    }
+    Ok(())
+}
 
-        // Delete hidden messages that are removed from the server.
+/// Removes from the database locally deleted messages that also don't
+/// have a server UID.
+pub fn prune_tombstones(context: &Context) -> sql::Result<()> {
     context.sql.execute(
         "DELETE FROM msgs \
          WHERE (chat_id = ? OR hidden) \
          AND server_uid = 0",
         params![DC_CHAT_ID_TRASH],
     )?;
-    }
     Ok(())
 }