Twenty Seventeen: Fix broken menu toggle in Customizer after menu items are added

This simplifies the line `$( siteNavigation.closest( '.main-navigation' ), this ).toggleClass( 'toggled-on' );` to `$( siteNavContain ).toggleClass( 'toggled-on' );`, since `this` is the clicked button, so the extra context isn't needed.

Props afercia, laurelfulford.

Fixes #38992.

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git-svn-id: https://core.svn.wordpress.org/trunk@39359 1a063a9b-81f0-0310-95a4-ce76da25c4cd