Remove signup flow experiment

This removes the last code from the signup flow experiment. The
confirmation after a succesful sign up will be handled by the
`soft_email_confirmation` feature.

app/controllers/registrations/welcome_controller.rb

@@ -51,7 +51,6 @@ module Registrations
     def requires_confirmation?(user)
       return false if user.confirmed?
       return false if Feature.enabled?(:soft_email_confirmation)
-      return false if experiment_enabled?(:signup_flow)
 
       true
     end

lib/gitlab/experimentation.rb

@@ -33,10 +33,6 @@ require 'zlib'
 module Gitlab
   module Experimentation
     EXPERIMENTS = {
-      signup_flow: {
-        tracking_category: 'Growth::Acquisition::Experiment::SignUpFlow',
-        use_backwards_compatible_subject_index: true
-      },
       onboarding_issues: {
         tracking_category: 'Growth::Conversion::Experiment::OnboardingIssues',
         use_backwards_compatible_subject_index: true

spec/lib/gitlab/experimentation_spec.rb

@@ -7,7 +7,6 @@ require 'spec_helper'
 RSpec.describe Gitlab::Experimentation::EXPERIMENTS do
   it 'temporarily ensures we know what experiments exist for backwards compatibility' do
     expected_experiment_keys = [
-      :signup_flow,
       :onboarding_issues,
       :ci_notification_dot,
       :upgrade_link_in_user_menu_a,