Remove inbox_watch option

Also resultified `get_watched_folders`.

Python test `test_moved_markseen` is modified to test using incoming
message instead of BCC-self message, because BCC-self message is
detected immediately in the Inbox.

src/config.rs

@@ -67,9 +67,6 @@ pub enum Config {
     #[strum(props(default = "1"))]
     MdnsEnabled,
 
-    #[strum(props(default = "1"))]
-    InboxWatch,
-
     #[strum(props(default = "1"))]
     SentboxWatch,