Removed invalid tests

Subject to discuss.
Image public properties Height, Width, Metadata and PixelType can't corrupt anything if backing image was disposed so I don't see any point altering that behaviour, it wasn't throwing before this branch and shouldn't throw after.

tests/ImageSharp.Tests/Image/ImageTests.cs

@@ -193,8 +193,6 @@ namespace SixLabors.ImageSharp.Tests
 
                 // Image<TPixel>
                 Assert.Throws<ObjectDisposedException>(() => { var prop = image.Frames; });
-                Assert.Throws<ObjectDisposedException>(() => { var prop = image.Metadata; });
-                Assert.Throws<ObjectDisposedException>(() => { var prop = image.PixelType; });
 
                 // Image
                 Assert.Throws<ObjectDisposedException>(() => { var prop = genericImage.Frames; });