change: deleted unused API v0 code

[jfcalvo]

Description

This PR deletes old code associated to API v0 including handlers, schemas, tests and some related code. Some small changes were necessary on API v1 to support these deletions.

I also used vulture to find some dead code.

There is still some code that could be deleted. I was not 100% sure but is very likely that we can still delete some code that is unused. We can take a look to it @frascuchon

Closes #4839

Type of change

(Please delete options that are not relevant. Remember to title the PR according to the type of change)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (change restructuring the codebase without changing functionality)
• Improvement (change adding some improvement to an existing functionality)
• Documentation update

How Has This Been Tested

(Please describe the tests that you ran to verify your changes. And ideally, reference tests)

• Deleted old tests and added small changes on the new ones.

Checklist

• I added relevant documentation
• follows the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I filled out the contributor form (see text above)
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)