Remove Book.AudioFormat property

This property was set to the highest quality returned by the library scan. Since adding quality option settings, it is no longer guaranteed to reflect the file that is downloaded. Also, the library scan qualities don't contain spatial audio or widevine-specific qualities., only ADRM.

Source/DataLayer/EfClasses/Book.cs

@@ -43,9 +43,9 @@ namespace DataLayer
         public ContentType ContentType { get; private set; }
         public string Locale { get; private set; }
 
-        internal AudioFormatEnum _audioFormat;
-
-        public AudioFormat AudioFormat { get => AudioFormat.FromEnum(_audioFormat); set => _audioFormat = value.ToEnum(); }
+        //This field is now unused, however, there is little sense in adding a
+        //database migration to remove an unused field. Leave it for compatibility.
+        internal long _audioFormat;
 
         // mutable
         public string PictureId { get; set; }