Widget Customizer: Remove WidgetCustomizer.showFirstSidebarIfRequested().

It's currently unused and needs another iteration.

see #27290.
Built from https://develop.svn.wordpress.org/trunk@27891


git-svn-id: http://core.svn.wordpress.org/trunk@27722 1a063a9b-81f0-0310-95a4-ce76da25c4cd