imap: don't try to connect in is_online()

Attempting to connect to the server when calling imap's is_online()
blocks the UI process, so don't.

melib/src/backends/imap.rs

@@ -185,9 +185,9 @@ pub(self) fn try_lock<T>(
 
 impl MailBackend for ImapType {
     fn is_online(&self) -> Result<()> {
-        if let Ok(mut g) = try_lock(&self.connection, None) {
-            let _ = g.connect();
-        }
+        //if let Ok(mut g) = try_lock(&self.connection, None) {
+        //    let _ = g.connect();
+        //}
         try_lock(&self.uid_store.is_online, None)?.1.clone()
     }