LineTreeGuide should not fall back to ASCII

If the user's environment didn't support unicode, we used
to fall back to using the AsciiTreeGuide if LineTreeGuide
was being used (which it is by default). This commit removes
that fallback since the characters used in LineTreeGuide is
covered by extended ASCII, which SHOULD be fine in almost all
scenarios.

Closes #324

src/Spectre.Console/Rendering/Tree/LineTreeGuide.cs

@@ -7,9 +7,6 @@ namespace Spectre.Console.Rendering
     /// </summary>
     public sealed class LineTreeGuide : TreeGuide
     {
-        /// <inheritdoc/>
-        public override TreeGuide? SafeTreeGuide => Ascii;
-
         /// <inheritdoc/>
         public override string GetPart(TreeGuidePart part)
         {