web.webmention_discover: discard HTTP response in returned endpoint

we don't use it and it's occasionally too big for memcache: https://console.cloud.google.com/errors/detail/CKLVg_O6-5vHpQE;time=PT6H;locations=global?inv=1&invt=AbyzyQ&project=bridgy-federated

web.py

@@ -1043,4 +1043,6 @@ def webmention_endpoint_cache_key(url):
 @memcache.memoize(expire=timedelta(hours=2), key=webmention_endpoint_cache_key)
 def webmention_discover(url, **kwargs):
     """Thin caching wrapper around :func:`oauth_dropins.webutil.webmention.discover`."""
-    return webmention.discover(url, **kwargs)
+    # discard the response since we don't use it and it's occasionally too big for
+    # memcache
+    return webmention.discover(url, **kwargs)._replace(response=None)