Remove unused field in EmbedFieldBuilder. (#1018)

src/Discord.Net.Core/Entities/Messages/EmbedBuilder.cs

@@ -231,7 +231,6 @@ namespace Discord
     {
         private string _name;
         private string _value;
-        private EmbedField _field;
         public const int MaxFieldNameLength = 256;
         public const int MaxFieldValueLength = 1024;