NetPlayProto: Remove now-unused MessageId type alias

Now that we have enum helpers for inserting values into packets and have
migrated all other enumerations over, there's no need to keep this alias
around any longer.

Source/Core/Core/NetPlayProto.h

@@ -122,6 +122,8 @@ struct NetTraversalConfig
 
 enum class MessageID : u8
 {
+  ConnectionSuccessful = 0,
+
   PlayerJoin = 0x10,
   PlayerLeave = 0x11,
 
@@ -225,7 +227,6 @@ struct WiimoteInput
   u8 report_id;
   std::vector<u8> data;
 };
-using MessageId = u8;
 using PlayerId = u8;
 using FrameNum = u32;
 using PadIndex = s8;

Source/Core/Core/NetPlayServer.cpp

@@ -431,9 +431,9 @@ ConnectionError NetPlayServer::OnConnect(ENetPeer* socket, sf::Packet& rpac)
   spac << player.pid << player.name << player.revision;
   SendToClients(spac);
 
-  // send new client success message with their id
+  // send new client success message with their ID
   spac.clear();
-  spac << static_cast<MessageId>(0);
+  spac << MessageID::ConnectionSuccessful;
   spac << player.pid;
   Send(player.socket, spac);