client/network: Remove option to disable yamux flow control (#7358)

With the `OnRead` flow control option yamux "send[s] window updates only
when data is read on the receiving end" and not as soon as "a Stream's
receive window drops to 0".

Yamux flow control has proven itself. This commit removes the feature
flag. Yamux flow control is now always enabled.

substrate/utils/browser/src/lib.rs

@@ -57,7 +57,6 @@ where
 		wasm_external_transport: Some(transport.clone()),
 		allow_private_ipv4: true,
 		enable_mdns: false,
-		use_yamux_flow_control: true,
 	};
 
 	let config = Configuration {