feat: use external deepin-pdfium and remove bundled submodule

[hillwoodroc]

deepin-pdfium has been continuously maintained and updated, so there is no need to bundle an older version inside the project.

deepin-pdfium 一直都在正常维护更新，为什么要内置一个更旧的版本而不是在项目里面链接 deepin-pdfium ？

Log: This fix replaces the built-in deepin-pdfium with the system-provided PkgConfig-based deepin-pdfium package.

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: hillwoodroc

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[lzwind]

因为兼容性的原因，不能合入。我们在最新版本需要同时支持Qt5与Qt6同时构建包。

[hillwoodroc]

为什么不在旧环境里面打包一个旧版本的 deepin-pdfium ？用内置死代码对新环境非常不友好。