Remove unused property

Was used in the save detection method which was redesigned to not need a
filename.

Saves/SAV6.cs

@@ -7,7 +7,6 @@ namespace PKHeX
     public sealed class SAV6 : SaveFile
     {
         // Save Data Attributes
-        public const string DefaultFileName = "main";
         public override string BAKName => $"{FileName} [{OT} ({Version}) - {LastSavedTime}].bak";
         public override string Filter => "Main SAV|*.*";
         public override string Extension => "";