Clear session['next'] if we are done

It will currently contain whatever it was last set to since the session
value is persistent across steps, but the way to signal to slixmpp that
we are done and it should send status=completed is to have next set to
None, per https://codeberg.org/poezio/slixmpp/src/branch/master/slixmpp/plugins/xep_0050/adhoc.py#L317

slixfeed/xmpp/client.py

@@ -876,6 +876,7 @@ class Slixfeed(slixmpp.ClientXMPP):
 
 
     async def _handle_import_complete(self, payload, session):
+        session['next'] = None
         url = payload['values']['url']
         if url.startswith('http') and url.endswith('.opml'):
             jid = session['from'].bare
@@ -896,17 +897,9 @@ class Slixfeed(slixmpp.ClientXMPP):
             except:
                 session['notes'] = [['error', 'Import failed. Filetype does not appear to be an OPML file.']]
                 session['has_next'] = False
-
-                # Mitigate Cheogram issue
-                # Gajim acts also strage, so this might be an issue with slixmmpp
-                # session['next'] = False
         else:
             session['notes'] = [['error', 'Import aborted. Send URL of OPML file.']]
             session['has_next'] = False
-
-            # Mitigate Cheogram issue
-            # Gajim acts also strage, so this might be an issue with slixmmpp
-            # session['next'] = False
         return session