Remove rate_limit_profile_update_username feature flag

Enforce rate limit of maximum 10 calls per minute
per logged-in user on changing username

Changelog: added

app/controllers/profiles_controller.rb

@@ -7,7 +7,7 @@ class ProfilesController < Profiles::ApplicationController
   before_action :user
   before_action :authorize_change_username!, only: :update_username
   before_action only: :update_username do
-    check_rate_limit!(:profile_update_username, scope: current_user) if Feature.enabled?(:rate_limit_profile_update_username, default_enabled: :yaml)
+    check_rate_limit!(:profile_update_username, scope: current_user)
   end
   skip_before_action :require_email, only: [:show, :update]
   before_action do

config/feature_flags/development/rate_limit_profile_update_username.yml

----
-name: rate_limit_profile_update_username
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77221
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/349132
-milestone: '14.7'
-type: development
-group: group::optimize
-default_enabled: false