Remove redundant connection_or_digest_is_known_relay() from CHANNEL_IS_CLIENT

After we fix #24898 (moved), we should fix this macro as well:

#define CHANNEL_IS_CLIENT(chan, options) \
  (!public_server_mode((options)) || channel_is_client(chan) || \
      !connection_or_digest_is_known_relay((chan)->identity_digest))

And we should rename it to something like CHANNEL_IS_CLIENT_AT_EITHER_END() to fix #22717 (moved).