Update compiler/src/dotty/tools/dotc/ast/Positioned.scala

Co-authored-by: Nicolas Stucki <nicolas.stucki@gmail.com>

Author: odersky

compiler/src/dotty/tools/dotc/ast/Positioned.scala

@@ -21,7 +21,6 @@ import java.io.{ PrintWriter }
 abstract class Positioned(implicit @constructorOnly src: SourceFile) extends SrcPos, Product, Cloneable {
   import Positioned.{ids, nextId, debugId}
 
-  private var myUniqueId: Int = _
   private var mySpan: Span = _
 
   /** A unique identifier in case -Yshow-tree-ids, or -Ydebug-tree-with-id