Don't set QA_IMAGE for dot-com

What does this MR do?

Resolves gitlab-org/quality/test-governance/request-for-help#39 (closed)

There was an incident, where the e2e tests were ahead of what was deployed in staging-canary

I noticed with !523 (diffs) the qa image is being set to registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa:master.

• Before !523 (merged): https://ops.gitlab.net/gitlab-org/quality/staging-canary/-/jobs/19118501#L41 -> registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa: 7d3b8e1863e
• After !523 (merged): https://ops.gitlab.net/gitlab-org/quality/staging-canary/-/jobs/19126257#L50 -> registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa:master

Check-list

• Ensure commits include Changelog: trailer
• Verify the change in all affected pipelines.