fix(ai): remove deprecated Hugging Face client

[InAnYan]

Fixes https://github.com/JabRef/jabref-issue-melting-pot/issues/1089

Hugging Face client was marked for removal, I removed it.

Now, to use Hugging Face LLM API you just need a different base URL. And it now uses OpenAI API. Old URL doesn't work anymore.

Steps to test

1. Select Hugging Face as provider
2. Chat

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• [/] I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[InAnYan]

I'm not sure, do I need to add a CHANGELOG.md entry?

[calixtus]

No changelog entry needed. Only for user facing changes.

[Siedlerchr]

PR was stuck somehow