Remove unmaintained `nsa` gem (#25265)

Gemfile

@@ -60,7 +60,6 @@ gem 'kaminari', '~> 1.2'
 gem 'link_header', '~> 0.0'
 gem 'mime-types', '~> 3.4.1', require: 'mime/types/columnar'
 gem 'nokogiri', '~> 1.15'
-gem 'nsa', '~> 0.2'
 gem 'oj', '~> 3.14'
 gem 'ox', '~> 2.14'
 gem 'parslet'

Gemfile.lock

@@ -442,11 +442,6 @@ GEM
     nokogiri (1.15.2)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
-    nsa (0.2.8)
-      activesupport (>= 4.2, < 7)
-      concurrent-ruby (~> 1.0, >= 1.0.2)
-      sidekiq (>= 3.5)
-      statsd-ruby (~> 1.4, >= 1.4.0)
     oj (3.14.3)
     omniauth (1.9.2)
       hashie (>= 3.4.6)
@@ -682,7 +677,6 @@ GEM
       net-scp (>= 1.1.2)
       net-ssh (>= 2.8.0)
     stackprof (0.2.25)
-    statsd-ruby (1.5.0)
     stoplight (3.0.1)
       redlock (~> 1.0)
     strong_migrations (0.8.0)
@@ -831,7 +825,6 @@ DEPENDENCIES
   net-http (~> 0.3.2)
   net-ldap (~> 0.18)
   nokogiri (~> 1.15)
-  nsa (~> 0.2)
   oj (~> 3.14)
   omniauth (~> 1.9)
   omniauth-cas (~> 2.0)

config/initializers/statsd.rb

@@ -1,15 +0,0 @@
-# frozen_string_literal: true
-
-if ENV['STATSD_ADDR'].present?
-  host, port = ENV['STATSD_ADDR'].split(':')
-
-  $statsd = ::Statsd.new(host, port)
-  $statsd.namespace = ENV.fetch('STATSD_NAMESPACE') { ['Mastodon', Rails.env].join('.') }
-
-  ::NSA.inform_statsd($statsd) do |informant|
-    informant.collect(:action_controller, :web)
-    informant.collect(:active_record, :db)
-    informant.collect(:active_support_cache, :cache)
-    informant.collect(:sidekiq, :sidekiq)
-  end
-end