Perf: do not need merge theme_config

[YunYouJun]

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?

Do not need merge theme_config, because hexo has merged it.

https://github.com/hexojs/hexo/blob/9b41e6d30bef0418d312e783c9bba6d290c1e4fb/lib/hexo/index.js#L41

[welcome]

Thanks so much for opening your first PR here!

[CLAassistant]

All committers have signed the CLA.

[stevenjoezhang]

See also hexojs/hexo#4360 (comment) . This is the "additional code"
Deleting it will cause bugs with Hexo 4, but Hexo 5 will not be affected

[YunYouJun]

I see. 🤣 So this is a compatible baggage.

[stevenjoezhang]

I will apply this patch after NexT drops Hexo 4 support. Thanks for your contribution!

Update: 817ef09

See also theme-next/hexo-theme-next#1430