Follow-up: Remove `allowed_to_fail` flag from the visual integration test job

!1296 (merged) marked the visual_integration_test job as "allowed to fail" to circumvent the following situation:

Currently, the GitLab UI CSS included in application.css overwrites the GitLab UI CSS coming from the GitLab UI repository. This situation has become a blocker to release other GitLab UI features.

We should disallow this job to fail once we address the CSS inclusion order.