Remove cluster applications usage data

What does this MR do and why?

GitLab managed apps have been removed. This removes leftover usage data tracking in preparation for backend model removal.

As part of this change, we also introduce tracking for the Prometheus and Elastic Stack cluster integrations.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Migration output

bin/rails db:migrate RAILS_ENV=test

== 20210915000453 AddIndexOnClustersIntegrationPrometheusEnabled: migrating ===
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:clusters_integration_prometheus, [:enabled, :created_at, :cluster_id], {:name=>"index_clusters_integration_prometheus_enabled", :algorithm=>:concurrently})
   -> 0.0023s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:clusters_integration_prometheus, [:enabled, :created_at, :cluster_id], {:name=>"index_clusters_integration_prometheus_enabled", :algorithm=>:concurrently})
   -> 0.0033s
-- execute("RESET statement_timeout")
   -> 0.0005s
== 20210915000453 AddIndexOnClustersIntegrationPrometheusEnabled: migrated (0.0101s) 

== 20210915001242 AddIndexOnClustersIntegrationElasticStackEnabled: migrating =
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:clusters_integration_elasticstack, [:enabled, :created_at, :cluster_id], {:name=>"index_clusters_integration_elasticstack_enabled", :algorithm=>:concurrently})
   -> 0.0009s
-- add_index(:clusters_integration_elasticstack, [:enabled, :created_at, :cluster_id], {:name=>"index_clusters_integration_elasticstack_enabled", :algorithm=>:concurrently})
   -> 0.0020s
== 20210915001242 AddIndexOnClustersIntegrationElasticStackEnabled: migrated (0.0042s) 

bin/rails db:rollback RAILS_ENV=test

== 20210930081208 FixDeprecatedApiThrottleDefaults: reverting =================
-- change_column_default(:application_settings, :throttle_authenticated_deprecated_api_period_in_seconds, {:from=>3600, :to=>1800})
   -> 0.0705s
-- change_column_default(:application_settings, :throttle_unauthenticated_deprecated_api_requests_per_period, {:from=>1800, :to=>3600})
   -> 0.0657s
== 20210930081208 FixDeprecatedApiThrottleDefaults: reverted (0.1402s) ========