melib/backends/notmuch: fix tag not being removed in set_flags()

May be related to #132

Cannot remove tags in the notmuch backend #132

> Running tag remove TAG on the notmuch backend does nothing. At a
> glance, this seems to be because NotmuchMailbox::set_flags never bothers
> to remove tags that are already present but not in the list of new tags.
> I could try fixing it, but I have no idea how the contribution process
> works here (my guess is the mailing list, but, well, #131).

https://git.meli.delivery/meli/meli/issues/132

melib/src/backends/notmuch.rs

@@ -769,7 +769,7 @@ impl MailBackend for NotmuchDb {
                         }
                         Err(tag) => {
                             let c_tag = CString::new(tag.as_str()).unwrap();
-                            add_tag!(&c_tag.as_ref());
+                            remove_tag!(&c_tag.as_ref());
                         }
                     }
                 }