Remove IsDefault from FontFamily

This property wasn't used so didn't need to be made internal.

src/Avalonia.Base/Media/FontFamily.cs

@@ -79,11 +79,6 @@ namespace Avalonia.Media
         /// <remarks>Key is only used for custom fonts.</remarks>
         public FontFamilyKey? Key { get; }
 
-        /// <summary>
-        /// Returns <c>True</c> if this instance is the system's default.
-        /// </summary>
-        public bool IsDefault => Name.Equals(DefaultFontFamilyName);
-
         /// <summary>
         /// Implicit conversion of string to FontFamily
         /// </summary>