Remove current Engineering MR Rate KPI from Handbook

Why is this change being made?

Background information in gitlab-org/quality/engineering-analytics/team-tasks#38.

After discussion among Engineering leadership, there is a proposal to remove the current Engineering-wide MR Rate KPI. The rationale is that the metric as currently defined isn't as meaningful as it should be, since much of Engineering isn't primarily focused on product MRs

Discussion is still needed on whether the current Engineering-wide MR rate KPI should be replaced with either department-specific MR rate PIs, or a MR rate KPI that combines specific departments only instead of all of Engineering.

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