Revert Config option

The change accidentally part of a previous commit

Author: odersky

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -161,7 +161,7 @@ object Config {
   final val showCompletions = false
 
   /** If set, enables tracing */
-  final val tracingEnabled = true
+  final val tracingEnabled = false
 
   /** Initial capacity of uniques HashMap.
    *  Note: This MUST BE a power of two to work with util.HashSet