fix: set is_chatmail during initial configuration

This was initially done in the IMAP loop
to set is_chatmail for existing users.
They should all have the setting configured
by now unless they install some very old backup.

Setting during the configuration is needed
for Delta Chat Desktop because it caches the value
internally:
<https://github.com/deltachat/deltachat-desktop/issues/6068>

src/scheduler.rs

@@ -431,14 +431,6 @@ pub async fn convert_folder_meaning(
 }
 
 async fn inbox_fetch_idle(ctx: &Context, imap: &mut Imap, mut session: Session) -> Result<Session> {
-    if !ctx.get_config_bool(Config::FixIsChatmail).await? {
-        ctx.set_config_internal(
-            Config::IsChatmail,
-            crate::config::from_bool(session.is_chatmail()),
-        )
-        .await?;
-    }
-
     // Update quota no more than once a minute.
     if ctx.quota_needs_update(session.transport_id(), 60).await
         && let Err(err) = ctx.update_recent_quota(&mut session).await