expression: fix unused code in util.go (pingcap#23301)

expression/util.go

@@ -654,44 +654,6 @@ func PopRowFirstArg(ctx sessionctx.Context, e Expression) (ret Expression, err e
 	return
 }
 
-// exprStack is a stack of expressions.
-type exprStack struct {
-	stack []Expression
-}
-
-// pop pops an expression from the stack.
-func (s *exprStack) pop() Expression {
-	if s.len() == 0 {
-		return nil
-	}
-	lastIdx := s.len() - 1
-	expr := s.stack[lastIdx]
-	s.stack = s.stack[:lastIdx]
-	return expr
-}
-
-// popN pops n expressions from the stack.
-// If n greater than stack length or n is negative, it pops all the expressions.
-func (s *exprStack) popN(n int) []Expression {
-	if n > s.len() || n < 0 {
-		n = s.len()
-	}
-	idx := s.len() - n
-	exprs := s.stack[idx:]
-	s.stack = s.stack[:idx]
-	return exprs
-}
-
-// push pushes one expression to the stack.
-func (s *exprStack) push(expr Expression) {
-	s.stack = append(s.stack, expr)
-}
-
-// len returns the length of th stack.
-func (s *exprStack) len() int {
-	return len(s.stack)
-}
-
 // DatumToConstant generates a Constant expression from a Datum.
 func DatumToConstant(d types.Datum, tp byte, flag uint) *Constant {
 	t := types.NewFieldType(tp)