Remove add_remove_ci_variable spec from E2E test bank

What does this MR do and why?

Describe in detail what your merge request does and why.

Removing add_remove_ci_variable_spec.rb as part of gitlab-org/quality&37 (closed)

Related to gitlab-org/quality/quality-engineering/team-tasks#1647 (closed)

Feature test to be added requested here #391637 (closed)

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