Remove OutlineItem::close

It was only used in the Outline destructor and in HtmlOutputDev but
there it was kind of pointless
11 jobs for master in 11 minutes and 49 seconds
Status Name Job ID Coverage
  Build
passed build #9932705

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passed build_android #9932710

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passed build_clang12_libcpp #9932706

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passed build_clazy_clang12 #9932709

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passed build_mingw64_fedora34 #9932708

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passed build_ubuntu_18_04 #9932707

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passed clang_format #9932704

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  Document
passed cpp_docs #9932712

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passed glib_docs #9932713

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passed qt5_docs #9932711

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  Publish
passed trigger_pages #9932714

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