[ETCM-355] Remove obsolete type alias

Author: lukasz-golebiewski

src/main/scala/io/iohk/ethereum/network/p2p/MessageDecoders.scala

@@ -107,7 +107,6 @@ object ETH63MessageDecoder extends MessageDecoder {
 
 // scalastyle:off
 object EthereumMessageDecoder {
-  type Decoder = (Int, Array[Byte]) => Message
   def ethMessageDecoder(protocolVersion: Capability): MessageDecoder =
     protocolVersion match {
       case Capability.Capabilities.Etc64Capability => ETC64MessageDecoder.orElse(NetworkMessageDecoder)