Remove remnant of old code

No longer any reason to handle delegates specifically : it will be done
naturally by the marshallerCache (and who knows, maybe it *can*
serialize some delegates !).
Moreover, this could misinterpret a signature as valid without parameter,
with somewhat weird consequences.

src/protobuf-net.Grpc/Internal/ContractOperation.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Reflection;
 using Grpc.Core;
@@ -179,8 +179,6 @@ static TypeCategory GetCategory(MarshallerCache marshallerCache, Type type, IBin
                 if (genType == typeof(AsyncServerStreamingCall<>)) return TypeCategory.AsyncServerStreamingCall;
             }
 
-            if (typeof(Delegate).IsAssignableFrom(type)) return TypeCategory.None; // yeah, that's not going to happen
-
             if (marshallerCache.CanSerializeType(type)) return TypeCategory.Data;
             bindContext?.LogWarning("Type cannot be serialized; ignoring: {0}", type.FullName);
             return TypeCategory.Invalid;
@@ -205,7 +203,7 @@ internal static bool TryIdentifySignature(MethodInfo method, BinderConfiguration
             if (method.IsGenericMethodDefinition) return false; // can't work with <T> methods
 
             if ((method.Attributes & (MethodAttributes.SpecialName)) != 0) return false; // some kind of accessor etc
-            
+
             if (!binderConfig.Binder.IsOperationContract(method, out var opName)) return false;
 
             var args = method.GetParameters();