lib: remove literalExpression and literalDocBook logic

3 years old, not needed anymore.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

modules/lib/stdlib-extended.nix

@@ -9,9 +9,5 @@ in
 nixpkgsLib.extend (
   self: super: {
     hm = mkHmLib { lib = self; };
-
-    # For forward compatibility.
-    literalExpression = super.literalExpression or super.literalExample;
-    literalDocBook = super.literalDocBook or super.literalExample;
   }
 )