Hide options for customizing the CDN URLs from _config.yml

[stevenjoezhang]

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).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

Issue resolved:

What is the new behavior?

• Link to demo site with this changes:
• Screenshots with this changes:

About a year ago a new method of configuring CDN URLs was introduced in #72
The method of manually setting CDN will be deprecated in the future, because it may mislead bloggers to set outdated or incompatible CDN URLs

How to use?

In NexT _config.yml:

Docs: next-theme/theme-next-docs@435d21b

[coveralls]

Coverage remained the same at 94.468% when pulling 946420b on cdn into 361c7e1 on master.