activitypub: drop japanese spam flood mitigation

this stopped (at least to BF) as of 2/24

activitypub.py

@@ -899,20 +899,6 @@ def inbox(protocol=None, id=None):
         logger.info('Dropping non-public activity')
         return 'OK'
 
-    # temporary, for Feb 2024 Japanese mention spam flood
-    # examples:
-    # https://nafo.uk/@usc1qrtm8s/111953165825053473
-    # https://kish.social/@kj2hevtalc/111954393290908177
-    if type == 'Create':
-        obj = activity.get('object', {})
-        mentions = [t for t in util.get_list(obj, 'tag') if t.get('type') == 'Mention']
-        if (len(mentions) in (4, 5)
-                and len(util.get_list(obj, 'attachment')) in (0, 1)
-                and not obj.get('inReplyTo')
-                and len(obj.get('attributedTo', '').split('/')[-1]) == 10):
-            logger.warning('Ignoring, looks like Feb 2024 Japanese mention spam flood')
-            return 'Ignoring, looks like Feb 2024 Japanese mention spam flood'
-
     if type == 'Follow':
         # rendered mf2 HTML proxy pages (in render.py) fall back to redirecting
         # to the follow's AS2 id field, but Mastodon's Accept ids are URLs that