Remove redundant with Checking

`ReTyper` already extends `Typer` which extends `Checking`

[Cherry-picked fcfa6b0]

Author: mbovel

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -206,7 +206,7 @@ object TreeChecker {
   private[TreeChecker] def isValidJVMMethodName(name: Name): Boolean = name.toString.forall(isValidJVMMethodChar)
 
 
-  class Checker(phasesToCheck: Seq[Phase]) extends ReTyper with Checking {
+  class Checker(phasesToCheck: Seq[Phase]) extends ReTyper {
     import ast.tpd.*
 
     protected val nowDefinedSyms = util.HashSet[Symbol]()