Revert "Proposal to change TextNormalizingTransformer.Mapper"

This reverts commit 19a2d0a.

src/Microsoft.ML.Transforms/Text/TextNormalizing.cs

@@ -398,8 +398,7 @@ private void NormalizeSrc(in ReadOnlyMemory<char> src, ref ReadOnlyMemory<char>
                 if (min == 0)
                 {
                     Host.Assert(src.Length == len);
-                    buffer.AppendSpan(span);
-                    dst = buffer.ToString().AsMemory();
+                    dst = src;
                 }
                 else
                 {