api!: make MsgId.delete_from_db() private

Use `delete_msgs()` if you are using the Delta Chat core
as a library and want to delete a message.

src/message.rs

@@ -113,7 +113,7 @@ WHERE id=?;
     }
 
     /// Deletes a message, corresponding MDNs and unsent SMTP messages from the database.
-    pub async fn delete_from_db(self, context: &Context) -> Result<()> {
+    pub(crate) async fn delete_from_db(self, context: &Context) -> Result<()> {
         // We don't use transactions yet, so remove MDNs first to make
         // sure they are not left while the message is deleted.
         context