Fix removed CI template metric definitions

What does this MR do and why?

When removing the templates, I forgot to mark the the corresponding metrics as removed. This is now blocking other updates to template metric definitions.

More precisely, when running the rake task to update CI templates known events, it removes p_ci_templates_serverless and p_ci_templates_managed_cluster_applications, which contribute to active metrics, which causes a SlotMismatch error in !86275 (merged) and !87000 (merged).

See also slack thread (internal link)

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How to set up and validate locally

Run, for example, the spec

bin/rspec spec/requests/api/usage_data_non_sql_metrics_spec.rb:24

With only the updates to lib/gitlab/usage_data_counters/known_events/ci_templates.yml, i.e. without marking the metrics as removed, the spec fails with SlotMismatchError

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