fix(merge): drop leftover <<<<<<< HEAD marker in jid_is_valid

My earlier merge-resolution Edit on src/xmpp/jid.c only covered the
=======/>>>>>>> half of the hunk and left an orphan <<<<<<< HEAD on
line 82 of jid_is_valid. CI caught it. Removed.

src/xmpp/jid.c

@@ -79,7 +79,6 @@ jid_create(const gchar* const str)
 gboolean
 jid_is_valid(const gchar* const str)
 {
-<<<<<<< HEAD
     if (str == NULL) {
         return FALSE;
     }