Remove use of database testing access token in ci

What does this MR do and why?

Describe in detail what your merge request does and why.

The CI variable GITLABCOM_DATABASE_TESTING_ACCESS_TOKEN isn't necessary to trigger downstream pipelines for the database testing pipeline. Instead the trigger token (GITLABCOM_DATABASE_TESTING_TRIGGER_TOKEN) is being used. The CI variable has already been removed from gitlab-org/gitlab's ci configuration, so this simply cleans up the code to no longer pass nil in a place where the value is never used.

How to set up and validate locally

I've triggered the db:gitlabcom-database-testing job on this MR. You can see that it worked by the comment that it posted.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.