Remove File extension check that only causes failure.

If you have a file named something like "button.down.png" and try load it with ```content.Load<Texture2D>("button.down")``` then we fail to load it as we detect it has an extension in the file name.

This check appears to be unnecessary as there is a check for the file existing a few lines above, also it differs from XNA behaviour, where files named like this work fine.