Removes mr_event_rule_pipeline spec

What does this MR do and why?

This E2E test is being removed as part of the Ops Section => End-to-end (e2e) test cleanup an... (gitlab-org/quality&37 - closed) initiative. This spec was identified as a candidate to be removed as it is a workflow that gets tested regularly already daily every time we create a new MR.

Related to gitlab-org/quality/quality-engineering/team-tasks#1649 (closed)

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