Remove empty sections in WPF articles

[adegeo]

Summary

Some of the WPF control reference had a bunch of empty sections at the end of the articles.

@BillWagner

Fixes #1877

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📄 File	🔗 Preview link
dotnet-desktop-guide/framework/wpf/advanced/drag-and-drop.md	dotnet-desktop-guide/framework/wpf/advanced/drag-and-drop
dotnet-desktop-guide/framework/wpf/advanced/element-tree-and-serialization-how-to-topics.md	dotnet-desktop-guide/framework/wpf/advanced/element-tree-and-serialization-how-to-topics
dotnet-desktop-guide/framework/wpf/advanced/events-how-to-topics.md	dotnet-desktop-guide/framework/wpf/advanced/events-how-to-topics
dotnet-desktop-guide/framework/wpf/advanced/globalization-and-localization.md	dotnet-desktop-guide/framework/wpf/advanced/globalization-and-localization
dotnet-desktop-guide/framework/wpf/advanced/input-and-commands-how-to-topics.md	dotnet-desktop-guide/framework/wpf/advanced/input-and-commands-how-to-topics
dotnet-desktop-guide/framework/wpf/advanced/migration-and-interoperability.md	dotnet-desktop-guide/framework/wpf/advanced/migration-and-interoperability
dotnet-desktop-guide/framework/wpf/controls/adorners-how-to-topics.md	dotnet-desktop-guide/framework/wpf/controls/adorners-how-to-topics
dotnet-desktop-guide/framework/wpf/controls/adorners.md	dotnet-desktop-guide/framework/wpf/controls/adorners
dotnet-desktop-guide/framework/wpf/controls/checkbox.md	dotnet-desktop-guide/framework/wpf/controls/checkbox
dotnet-desktop-guide/framework/wpf/controls/contextmenu.md	dotnet-desktop-guide/framework/wpf/controls/contextmenu
dotnet-desktop-guide/framework/wpf/controls/expander.md	dotnet-desktop-guide/framework/wpf/controls/expander
dotnet-desktop-guide/framework/wpf/controls/gridsplitter-how-to-topics.md	dotnet-desktop-guide/framework/wpf/controls/gridsplitter-how-to-topics
dotnet-desktop-guide/framework/wpf/controls/gridsplitter.md	dotnet-desktop-guide/framework/wpf/controls/gridsplitter
dotnet-desktop-guide/framework/wpf/controls/groupbox.md	dotnet-desktop-guide/framework/wpf/controls/groupbox
dotnet-desktop-guide/framework/wpf/controls/listbox-how-to-topics.md	dotnet-desktop-guide/framework/wpf/controls/listbox-how-to-topics
dotnet-desktop-guide/framework/wpf/controls/listbox.md	dotnet-desktop-guide/framework/wpf/controls/listbox
dotnet-desktop-guide/framework/wpf/controls/menu.md	dotnet-desktop-guide/framework/wpf/controls/menu
dotnet-desktop-guide/framework/wpf/controls/popup.md	dotnet-desktop-guide/framework/wpf/controls/popup
dotnet-desktop-guide/framework/wpf/controls/progressbar.md	dotnet-desktop-guide/framework/wpf/controls/progressbar
dotnet-desktop-guide/framework/wpf/controls/radiobutton.md	dotnet-desktop-guide/framework/wpf/controls/radiobutton
dotnet-desktop-guide/framework/wpf/controls/repeatbutton.md	dotnet-desktop-guide/framework/wpf/controls/repeatbutton
dotnet-desktop-guide/framework/wpf/controls/statusbar.md	dotnet-desktop-guide/framework/wpf/controls/statusbar
dotnet-desktop-guide/framework/wpf/controls/tabcontrol.md	dotnet-desktop-guide/framework/wpf/controls/tabcontrol
dotnet-desktop-guide/framework/wpf/controls/toolbar.md	dotnet-desktop-guide/framework/wpf/controls/toolbar
dotnet-desktop-guide/framework/wpf/controls/treeview-how-to-topics.md	dotnet-desktop-guide/framework/wpf/controls/treeview-how-to-topics

[BillWagner]

This LGTM @adegeo

Let's