Remove identity_verification feature flag

.rubocop_todo/rspec/feature_category.yml

.rubocop_todo/rspec/named_subject.yml

doc/development/identity_verification.md

ee/app/models/concerns/identity_verifiable.rb

...onfig/feature_flags/development/identity_verification.yml

ee/lib/gitlab/email/feature_flag_wrapper.rb

ee/spec/features/registrations/email_confirmation_spec.rb

ee/spec/features/registrations/identity_verification_spec.rb

ee/spec/lib/gitlab/email/feature_flag_wrapper_spec.rb

ee/spec/models/concerns/identity_verifiable_spec.rb

ee/spec/requests/ee/registrations_controller_spec.rb

ee/spec/requests/trial_registrations_controller_spec.rb

...rs/registrations_identity_verification_controller_spec.rb

ee/spec/support/helpers/saas_registration_helpers.rb

ee/spec/support/helpers/stub_saas_features.rb

...ort/shared_contexts/saas_registration_settings_context.rb

ee/spec/support_specs/stub_saas_features_spec.rb

spec/controllers/confirmations_controller_spec.rb

spec/controllers/registrations_controller_spec.rb

spec/features/users/login_spec.rb