Remove unused code from toolbar_pre_render_item that throws a warning on custom themed tabs

Problem/Motivation

While working on #1741498: Add a responsive preview bar to Drupal core, I noticed that passing a custom render array to toolbar for a tab raised the following warning:

Notice: Undefined index: #options in toolbar_pre_render_item() (line 291 of core/modules/toolbar/toolbar.module).

This warning is created by the following code in toolbar_pre_render_item

// Attributes are nested in the #options property passed to l().
// To make the return structure of toolbar_pre_render_item() uniform,
// attributes will always be returned in the #attributes property. These
// properties will be included in the #options parameter passed to l() in
// theme_toolbar_tab().
$element['#attributes'] = $element['tab']['#options']['attributes'];

This code is left over from the original hook_toolbar implementation that expected a tab to be structured like a link. Now that #1847198: Update the structure returned by hook_toolbar() is committed, we can remove the vestigial code.

Proposed resolution

Remove the unused code that is throwing a warning.