Remove gitlab_experiment_middleware feature flag

This adds the gitlab-experiment middleware, which currently handles
anonymous url redirects for experiments.

Changelog: added

config/feature_flags/development/gitlab_experiment_middleware.yml

----
-name: gitlab_experiment_middleware
-introduced_by_url:
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/323643
-milestone: '14.1'
-type: development
-group: group::adoption
-default_enabled: false

config/initializers/gitlab_experiment.rb

@@ -7,17 +7,3 @@ Gitlab::Experiment.configure do |config|
     pool: ->(&block) { Gitlab::Redis::SharedState.with { |redis| block.call(redis) } }
   )
 end
-
-# TODO: This shim should be removed after the feature flag is rolled out, as
-#   it only exists to facilitate the feature flag control of the behavior.
-module Gitlab::Experiment::MiddlewareWithFeatureFlags
-  attr_reader :app
-
-  def call(env)
-    return app.call(env) unless Feature.enabled?(:gitlab_experiment_middleware)
-
-    super
-  end
-end
-
-Gitlab::Experiment::Middleware.prepend(Gitlab::Experiment::MiddlewareWithFeatureFlags)