Remove NewRelic integration.

Remove the NewRelic integration in preparation for migrating to Elastic APM instead.

config/application.rb

@@ -49,9 +49,9 @@ module Danbooru
     config.active_record.schema_format = :sql
     config.encoding = "utf-8"
 
-    # Hide sensitive model attributes and request params in exception messages,
-    # log files, and in NewRelic. These are substring matches, so they match
-    # any attribute or request param containing the word 'password' etc.
+    # Hide sensitive model attributes and request params in exception messages
+    # and logs. These are substring matches, so they match any attribute or
+    # request param containing the word 'password' etc.
     #
     # https://guides.rubyonrails.org/configuring.html#config-filter-parameters
     config.filter_parameters += [:password, :api_key, :secret, :ip_addr, :address, :email_verification_key, :signed_user_id] if Rails.env.production?

config/danbooru_default_config.rb

@@ -547,12 +547,6 @@ module Danbooru
       # "redis://localhost:6379"
     end
 
-    # Optional. The license key for your New Relic account.
-    # https://newrelic.com/
-    # https://docs.newrelic.com/docs/accounts/accounts-billing/account-setup/new-relic-license-key/
-    def new_relic_license_key
-    end
-
     # True if the Winter Sale is active.
     def is_promotion?
       false

config/initializers/00_newrelic.rb

@@ -1,15 +0,0 @@
-if Danbooru.config.new_relic_license_key.present?
-  require "new_relic/control"
-
-  # https://github.com/newrelic/newrelic-ruby-agent/blob/1ef4082fe97fd19aeccb3f392a44ef3becae66d3/lib/newrelic_rpm.rb#L40
-  # https://github.com/newrelic/newrelic-ruby-agent/blob/1ef4082fe97fd19aeccb3f392a44ef3becae66d3/lib/new_relic/agent.rb#L349
-  NewRelic::Control.instance.init_plugin(
-    app_name: Danbooru.config.canonical_app_name,
-    license_key: Danbooru.config.new_relic_license_key,
-    log_level: Danbooru.config.debug_mode ? "debug" : "error",
-    #log: Rails.logger,
-    "rake.tasks": ["maintenance:.*"],
-    "browser_monitoring.auto_instrument": false,
-    config: Rails.application.config,
-  )
-end