SuccessorType: Address review comments.

ruby/ql/lib/codeql/ruby/controlflow/internal/Splitting.qll

@@ -122,12 +122,9 @@ module EnsureSplitting {
    * the `ensure` block must end with a `return` as well (provided that
    * the `ensure` block executes normally).
    */
-  class EnsureSplitType instanceof SuccessorType {
+  class EnsureSplitType extends SuccessorType {
     EnsureSplitType() { not this instanceof ConditionalSuccessor }
 
-    /** Gets a textual representation of this successor type. */
-    string toString() { result = super.toString() }
-
     /** Holds if this split type matches entry into an `ensure` block with completion `c`. */
     predicate isSplitForEntryCompletion(Completion c) {
       if c instanceof NormalCompletion