E2E tests: remove QA_DEFAULT_BRANCH env var usage

What does this MR do and why?

Resolves gitlab-org/quality/quality-engineering/team-tasks#3485 (closed)

Remove QA_DEFAULT_BRANCH env variable as it is no longer necessary to change the default branch via env variables. It currently can cause confusion as staging is set to master whereas MRs default to main.

Tests can be created with whichever default branch is required.

References

Related TF removal: https://gitlab.com/gitlab-org/quality/engineering-productivity-infrastructure/-/merge_requests/1054

Screenshots or screen recordings

NOTE: Staging / staging canary pipelines are the only pipelines left to use this variable, I have updated them to use main to ensure no tests fail:

After this MR is merged I will remove the variables from staging/staging-canary

How to set up and validate locally

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