Remove rate_limit_username_exists_endpoint feature flag

Enforce max 20 calls per mintue per IP address on
internal API endpoint /users/:username/exists,
used by the registration to perform a client-side
validation for uniqueness of the chosen username

Changelog: added

app/controllers/users_controller.rb

@@ -24,7 +24,7 @@ class UsersController < ApplicationController
   before_action :authorize_read_user_profile!,
                 only: [:calendar, :calendar_activities, :groups, :projects, :contributed, :starred, :snippets, :followers, :following]
   before_action only: [:exists] do
-    check_rate_limit!(:username_exists, scope: request.ip) if Feature.enabled?(:rate_limit_username_exists_endpoint, default_enabled: :yaml)
+    check_rate_limit!(:username_exists, scope: request.ip)
   end
 
   feature_category :users

config/feature_flags/development/rate_limit_username_exists_endpoint.yml

----
-name: rate_limit_username_exists_endpoint
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77119
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/348974
-milestone: '14.7'
-type: development
-group: group::optimize
-default_enabled: false