Remove unused internal constructor and its test

It was only used by that test.

Terminal.Gui/Drawing/Attribute.cs

@@ -44,15 +44,6 @@ public readonly record struct Attribute : IEqualityOperators<Attribute, Attribut
         Background = attr.Background;
     }
 
-    /// <summary>Initializes a new instance with platform specific color value.</summary>
-    /// <param name="platformColor">Value.</param>
-    internal Attribute (int platformColor)
-    {
-        PlatformColor = platformColor;
-        Foreground = Default.Foreground;
-        Background = Default.Background;
-    }
-
     /// <summary>Initializes a new instance of the <see cref="Attribute"/> struct.</summary>
     /// <param name="platformColor">platform-dependent color value.</param>
     /// <param name="foreground">Foreground</param>

UnitTests/Drawing/AttributeTests.cs

@@ -344,19 +344,6 @@ public class AttributeTests
         Assert.False (attr2.Equals (attr1));
     }
 
-    [Fact]
-    public void PlatformColorConstructor ()
-    {
-        // Arrange & Act
-        var attribute = new Attribute (42);
-
-        // Assert
-        //Assert.True (attribute.Initialized);
-        Assert.Equal (42, attribute.PlatformColor);
-        Assert.Equal (new Color (Color.White), attribute.Foreground);
-        Assert.Equal (new Color (Color.Black), attribute.Background);
-    }
-
     [Fact]
     public void ToString_ShouldReturnFormattedStringWithForegroundAndBackground ()
     {