refactor(Purchase): remove unused methods.

tienda_ilusion/don_confiao/frontend/don-confiao/src/components/Purchase.vue

@@ -150,7 +150,6 @@
        valid: false,
        client_search: '',
        product_search: '',
-       payment_cash: 0,
        payment_methods: null,
        purchase: {
          date: this.getCurrentDate(),
@@ -186,23 +185,6 @@
          return total + this.calculateSubtotal(saleline);
        }, 0);
      },
-     calculateChange() {
-       if (this.purchase.payment_method === 'CASH') {
-         return (this.payment_cash || 0) - this.calculateTotal;
-       } else {
-         return 0;
-       }
-     },
-     isValidPayment() {
-       if (this.calculateTotal === 0) {
-         return false;
-       }
-       if (this.purchase.payment_method === 'CASH' && this.payment_cash < this.calculateTotal) {
-         return false;
-       }
-       return true;
-     },
-
      filteredClients() {
        return this.clients.filter(client => {
          if (this.client_search === '') {