p2p: refactor channel description (#7130)

This is another small sliver of #7075, with the intention of removing
the legacy shim layer related to channel registration.

internal/blocksync/reactor.go

@@ -29,14 +29,13 @@ var (
 	// ref: https://github.com/tendermint/tendermint/issues/5670
 	ChannelShims = map[p2p.ChannelID]*p2p.ChannelDescriptorShim{
 		BlockSyncChannel: {
-			MsgType: new(bcproto.Message),
 			Descriptor: &p2p.ChannelDescriptor{
 				ID:                  byte(BlockSyncChannel),
+				MessageType:         new(bcproto.Message),
 				Priority:            5,
 				SendQueueCapacity:   1000,
 				RecvBufferCapacity:  1024,
 				RecvMessageCapacity: MaxMsgSize,
-				MaxSendBytes:        100,
 			},
 		},
 	}