refactor(useImport): 简化图片导出状态的处理逻辑

移除冗余的getImgExporting方法，直接使用imgExporting函数返回状态值

src/hooks/useImport.ts

@@ -227,8 +227,7 @@ export default () => {
     const win = window as any
     if (!win.exportJSON) win.exportJSON = exportJSON2
     if (!win.readJSON) win.readJSON = readJSON
-    if (!win.imgExporting) win.imgExporting = imgExporting.value
-    if (!win.getImgExporting) win.getImgExporting = () => imgExporting.value
+    if (!win.imgExporting) win.imgExporting = () => imgExporting.value
   }
 
   // 导入pptist文件