remove unneeded code

Author: SethTisue

docs/2023/puzzles/day19.md

@@ -196,8 +196,6 @@ Using these helpers, we can implement the abstract evaluator as follows:
 ```scala
 def part2(input: String): Long = combinations(input, 4001)
 
-extension [T](part: (T, T)) private inline def swap: (T, T) = (part._2, part._1)
-
 def combinations(input: String, until: Long): Long =
   val Array(workflowLines, _) = input.split("\n\n")
   val workflow = Workflow.parse(workflowLines.trim())