Remove unnecessary AI dependencies

[InAnYan]

Removed unnecessary AiService (it turns out there is no that much code, I used InelliJ analysis tools). Also removed AiApiKeyProvider, the code for API keys is now in the AiPreferences.

Mandatory checks

- [ ] Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
- [ ] Tests created for changes (if applicable)

• Manually tested changed features in running JabRef (always required)
  - [ ] Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked 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 to the documentation repository.