Remove the old REST code

[KristianWEB]

Is your feature request related to a problem? Please describe.
We need to remove the old REST code because it got replaced by GraphQL.

You should do this issue after you migrate the Posts REST API #55 #56. Until that you can use the code as a reference for faster implementation.

You can work on this issue by creating and using a branch called feature/remove-rest

Pull Request Squashing

Please, go through these steps before you submit a PR.

1. Make sure that your PR is not a duplicate.
2. If not, then make sure that:

2.1. You have done your changes in a separate branch. Branches MUST have descriptive names that start with either the fix/ or feature/ prefixes. Good examples are: fix/signin-issue or feature/issue-templates.

2.2. You have a descriptive commit message with a short title (first line).

2.3. You have only one commit (if not, squash them into one commit).

2.4. npm test doesn't throw any error. If it does, fix them first and amend your commit (git commit --amend).

3. After these steps, you're ready to open a pull request.

3.1. Give a descriptive title to your PR.

3.2. Provide a description of your changes.

3.3. Put closes #XXXX in your comment to auto-close the issue that your PR fixes (if such).