Remove legacy contextual sidebar styles

What does this MR do and why?

Since the introduction of the new navigation (.super-sidebar) the contextual sidebar has become redundant. Some legacy styles are still included in our stylesheet bundle, along with SCSS variables which use legacy colors/styles.

This MR removes the contextual sidebar styles, as well as cleaning up other HTML elements using classes within contextual_sidebar.scss and removing legacy spec files.

Removals:

• Remove .toggle-sidebar-button styles from bootstrap migration styles
• Remove contextual_sidebar.scss and contextual_sidebar_header.scss
• Remove .nav-sidebar class from print stylesheet
• Remove $contextual-sidebar SCSS variables
• Remove $nav-active-bg SCSS variable
• Remove .mobile-overlay empty <div> on page templates
• Remove redundant condition for modifySidebarPercentage method in learn_gitlab.vue
• Remove redundant spec tests

Updates:

• Update MR review footer to use $super-sidebar-width for left position
• Update boards styles to use $super-sidebar-width for left position
• Update usage of collapse-text class to use Tailwind utility classes

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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How to set up and validate locally

1. Pipeline should pass
2. MR review footer should display in correct position
3. Boards should display correctly