移除PayPal支付功能，仅保留支付宝支付

- 删除PayPalService.java和PayPalController.java
- 从PaymentMethod枚举中移除PAYPAL选项
- 移除PaymentController和PaymentApiController中的PayPal相关代码
- 移除前端PayPal支付选项和相关API
- 清理配置文件中的PayPal配置
- 修复OrderController中的PayPal引用错误

demo/src/main/resources/templates/payment/form.html

@@ -52,15 +52,6 @@
                                         </div>
                                     </div>
                                 </div>
-                                <div class="col-md-6">
-                                    <div class="card payment-card h-100" onclick="selectPaymentMethod('PAYPAL')">
-                                        <div class="card-body text-center">
-                                            <i class="fab fa-paypal payment-icon paypal-icon"></i>
-                                            <h5 class="card-title">PayPal</h5>
-                                            <p class="card-text">全球领先的在线支付</p>
-                                        </div>
-                                    </div>
-                                </div>
                             </div>
                             
                             <div class="mb-3">
@@ -128,9 +119,6 @@
             if (method === 'ALIPAY') {
                 currencySelect.value = 'CNY';
                 currencySelect.options[0].selected = true;
-            } else if (method === 'PAYPAL') {
-                currencySelect.value = 'USD';
-                currencySelect.options[1].selected = true;
             }
         }