Deprecate generate test session

What does this MR do and why?

Deprecates generate-test-session job from test-on-omnibus, test-on-omnibus-nightly and test-on-gdk. These sessions are stored at https://gitlab.com/gitlab-org/quality/testcase-sessions.

We are consolidating the reporting under one solution. This avoids duplication and extra maintenance, removing 2 minutes and 30 seconds (avg) of additional CI time. This was a custom-made solution previous to the Allure report which now provides the same information.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.