Remove gitment support

[stevenjoezhang]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• 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

What kind of change does this PR introduce?

• 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?

Issue resolved: #936

What is the new behavior?

• Screenshots with this changes: N/A
• Link to demo site with this changes: N/A

How to use?

In NexT _config.yml:

-gitment:
-  enable: false
-  mint: true # RECOMMEND, A mint on Gitment, to support count, language and proxy_gateway
-  count: true # Show comments count in post meta area
-  lazy: false # Comments lazy loading with a button
-  cleanly: false # Hide 'Powered by ...' on footer, and more
-  language: # Force language, or auto switch by theme
-  github_user: # MUST HAVE, Your Github Username
-  github_repo: # MUST HAVE, The name of the repo you use to store Gitment comments
-  client_id: # MUST HAVE, Github client id for the Gitment
-  client_secret: # EITHER this or proxy_gateway, Github access secret token for the Gitment
-  proxy_gateway: # Address of api proxy, See: https://github.com/aimingoo/intersect
-  redirect_protocol: # Protocol of redirect_uri with force_redirect_protocol when mint enabled

Does this PR introduce a breaking change?

• Yes.
• No.