C++: Avoid extends Operation in LeapYear.qll

The `Operation` class is abstract, and extending it caused cached stages
to be recomputed all the way down to the AST. This meant that the leap
year queries evaluated their own copy of SSA and data flow.

cpp/ql/src/Likely Bugs/Leap Year/LeapYear.qll

@@ -30,7 +30,7 @@ private predicate additionalLogicalCheck(Expr e, string operation, int valueToCh
 /**
  * An `Operation` that seems to be checking for leap year.
  */
-class CheckForLeapYearOperation extends Operation {
+class CheckForLeapYearOperation extends Expr {
   CheckForLeapYearOperation() {
     exists(BinaryArithmeticOperation bo | bo = this |
       bo.getAnOperand().getValue().toInt() = 4 and
@@ -39,8 +39,6 @@ class CheckForLeapYearOperation extends Operation {
       additionalLogicalCheck(this.getEnclosingElement(), "%", 400)
     )
   }
-
-  override string getOperator() { result = "LeapYearCheck" }
 }
 
 /**