fix: remove dead DPrinter construction in popupPrintDialog

1. Root cause: popupPrintDialog() constructs a DPrinter local
   variable (QPrinter::HighResolution) that is never used; its
   constructor synchronously loads the cups plugin and enumerates
   printers, adding avoidable latency to print preview
2. Fix: remove the unused DPrinter printer(QPrinter::HighResolution)
   line introduced by commit 9fcf3e4; the actual print dialog uses
   DPrintPreviewDialog with no arguments and builds its own printer
3. Impact: print preview opens faster by eliminating one redundant
   synchronous printer enumeration; no behavior change for valid
   print paths; no -Wunused-variable warning after removal

Log: Remove dead DPrinter construction to speed up print preview

Influence:
1. Test print preview with a syntax-highlighted file
2. Test print preview with a plain text file
3. Verify no unused-variable warnings in build output

fix: 删除 popupPrintDialog 中无用的 DPrinter 构造

1. 根因：popupPrintDialog() 构造了一个从未使用的 DPrinter 局部
   变量（QPrinter::HighResolution），其构造函数同步加载 cups
   插件并枚举打印机，为打印预览增加不必要的延迟
2. 方案：删除由 commit 9fcf3e4 引入的无用 DPrinter printer(
   QPrinter::HighResolution) 行；实际打印对话框使用无参
   DPrintPreviewDialog 并在内部自建 printer
3. 影响：消除一次冗余的同步打印机枚举，打印预览打开更快；
   对有效打印路径无行为变化；删除后无 -Wunused-variable 告警

Log: 删除无用 DPrinter 构造以加速打印预览

Influence:
1. 测试语法高亮文件的打印预览
2. 测试纯文本文件的打印预览
3. 验证编译输出无 unused-variable 告警

PMS: BUG-375053

Author: pengfeixx

src/widgets/window.cpp

@@ -2441,7 +2441,6 @@ void Window::popupPrintDialog()
         return;
     }
 
-    DPrinter printer(QPrinter::HighResolution);
     m_isNewPrint = true;
     m_pPreview = new DPrintPreviewDialog(this);
     m_pPreview->setAttribute(Qt::WA_DeleteOnClose);