Update index.md

Remove value-less keys

Author: shaedrich

topics/cst/index.md

@@ -1,11 +1,8 @@
 ---
-created_by: 
 display_name: Concrete syntax tree
-released: 
 short_description: A concrete syntax tree is a low level representation of the parsed source in the structure defined by a grammar description.
 topic: cst
 aliases: parse-tree, parsing-tree, derivation-tree
-url:
 wikipedia_url: https://en.wikipedia.org/wiki/Parse_tree
 related: ast
 ---