refactor(box): remove Bluebird.asCallback

[SukkaW]

What does it do?

Remove the callback from box.

All hexo internal functions are not relied on box's callback, and box's callback usage is never mentioned in Hexo API documents: https://hexo.io/api/box

How to test

git clone -b reduce-ascallback-box https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage remained the same at 97.822% when pulling a6c44c3 on SukkaW:reduce-ascallback-box into 7ff1a70 on hexojs:master.