refactor(check_metadata): remove check metadata loading

[pedrooot]

Description

Refactor code related with check metadata and custom checks metadata.

Checklist

• Are there new checks included in this PR? Yes / No
  • If so, do we need to update permissions for the provider? Please review this carefully.
• [x] Review if the code is being covered by tests.
• [x] Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• [x] Review if backport is needed.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.