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/frontend/src/views/PaymentCreate.vue

@@ -49,18 +49,6 @@
               <el-icon><CreditCard /></el-icon>
               支付宝
             </el-radio>
-            <el-radio value="PAYPAL">
-              <el-icon><CreditCard /></el-icon>
-              PayPal
-            </el-radio>
-            <el-radio value="WECHAT">
-              <el-icon><CreditCard /></el-icon>
-              微信支付
-            </el-radio>
-            <el-radio value="UNIONPAY">
-              <el-icon><CreditCard /></el-icon>
-              银联支付
-            </el-radio>
           </el-radio-group>
         </el-form-item>
 
@@ -103,30 +91,6 @@
             <p>支持支付宝扫码支付和网页支付</p>
           </div>
         </el-col>
-        
-        <el-col :xs="24" :sm="12" :md="6">
-          <div class="payment-method-info">
-            <el-icon size="32" color="#0070BA"><CreditCard /></el-icon>
-            <h5>PayPal</h5>
-            <p>支持PayPal账户支付和信用卡支付</p>
-          </div>
-        </el-col>
-        
-        <el-col :xs="24" :sm="12" :md="6">
-          <div class="payment-method-info">
-            <el-icon size="32" color="#07C160"><CreditCard /></el-icon>
-            <h5>微信支付</h5>
-            <p>支持微信扫码支付和H5支付</p>
-          </div>
-        </el-col>
-        
-        <el-col :xs="24" :sm="12" :md="6">
-          <div class="payment-method-info">
-            <el-icon size="32" color="#E6A23C"><CreditCard /></el-icon>
-            <h5>银联支付</h5>
-            <p>支持银联卡支付和网银支付</p>
-          </div>
-        </el-col>
       </el-row>
     </el-card>
   </div>