Remove undocumented page

We're not keeping this page up to date, so it's only a small
portion of undocumented content. This is misleading because it
makes readers think that this is the only information not
documented.

Author: Havvy

src/SUMMARY.md

@@ -123,5 +123,4 @@
 - [Appendices](appendices.md)
     - [Macro Follow-Set Ambiguity Formal Specification](macro-ambiguity.md)
     - [Influences](influences.md)
-    - [As-Yet-Undocumented Features](undocumented.md)
     - [Glossary](glossary.md)