fix: remove TaskType.multi_task_text_token_classification

[alvarobartt]

Description

This PR removes the multi_task_text_token_classification from the TaskType enum from both the client and the server, as it's unused, and can lead to confusion.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested

• No need to, as TaskType.multi_task_text_token_classification was not being covered by tests

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/)

[github-actions]

The URL of the deployed environment for this PR is https://argilla-quickstart-pr-3640-ki24f765kq-no.a.run.app

[frascuchon]

Even if this is something useless, it would be nice to track the change in the CHANGELOG.md inside the Removed section