Remove box-shadow of .sidebar when back2top.sidebar is true

[backrunner]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

The back2top button can't apply the shadow, it looks uncomfortable.

What is the new behavior?

I removed the shadow of sidebar when back2top.sidebar is true.

• Screenshots with this changes:
  

Does this PR introduce a breaking change?

• Yes.
• No.