fix: Fixes thread guard and cleans up incoming packet reader (#1641)

### Summary
* Fixes syntax compile error when THREADGUARD is enabled.
* Removes `int packetLength` from incoming packet handles since they aren't needed.

### Developer Notes
Incoming packet handler `SpanReader` is now properly scoped to that packet by length.

Projects/UOContent/Network/ContainerGridPacketHandler.cs

@@ -20,7 +20,7 @@ namespace Server.Network;
 public unsafe class ContainerGridPacketHandler : PacketHandler
 {
     public ContainerGridPacketHandler(int packetID, int length, bool ingame,
-        delegate*<NetState, SpanReader, int, void> onReceive)
+        delegate*<NetState, SpanReader, void> onReceive)
         : base(packetID, length, ingame, onReceive)
     {
     }