Fix deploy token API to delete underlying deploy_token record

What does this MR do?

The deploy token API DELETE endpoints currently find a deploy token like:

deploy_token = user_group.group_deploy_tokens.find_by_deploy_token_id!(params[:token_id])

Then runs deploy_token.destroy. But that runs destroy on the group_deploy_token record which has the deploy_token_id foreign key instead of the deploy_token record, so in order for the cascade delete to work, we need the destroy to be on the deploy_token.

This MR updates the API to destroy the deploy_token record so both associated records are properly removed.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related #212523 (closed)