`sync` protocol now can have negotiated fallback name (#11982)

substrate/client/network/src/protocol.rs

@@ -1577,8 +1577,6 @@ where
 			} => {
 				// Set number 0 is hardcoded the default set of peers we sync from.
 				if set_id == HARDCODED_PEERSETS_SYNC {
-					debug_assert!(negotiated_fallback.is_none());
-
 					// `received_handshake` can be either a `Status` message if received from the
 					// legacy substream ,or a `BlockAnnouncesHandshake` if received from the block
 					// announces substream.