Remove importer_user_mapping FF check from controller and job

The endpoints users use to accept or reject user mapping can't be behind
the actor-based importer_user_mapping feature flag because we don't
control which user group owners will reassign the contributions to.
So, these endpoints need to be available to all users.

Also the job can be enable/disable via Sidekiq Worker feature flags if
needed

app/controllers/import/source_users_controller.rb

@@ -4,7 +4,6 @@ module Import
   class SourceUsersController < ApplicationController
     prepend_before_action :check_feature_flag!
 
-    before_action :source_user
     before_action :check_source_user_valid!
 
     respond_to :html
@@ -53,7 +52,7 @@ def source_user
     strong_memoize_attr :source_user
 
     def check_feature_flag!
-      not_found unless Feature.enabled?(:importer_user_mapping, current_user)
+      not_found unless Feature.enabled?(:importer_user_mapping, source_user.reassigned_by_user)
     end
 
     def banner(partial)

app/workers/import/reassign_placeholder_user_records_worker.rb

@@ -19,11 +19,6 @@ class ReassignPlaceholderUserRecordsWorker
     def perform(import_source_user_id, _params = {})
       @import_source_user = Import::SourceUser.find_by_id(import_source_user_id)
 
-      return unless Feature.enabled?(
-        :importer_user_mapping,
-        User.actor_from_id(import_source_user&.reassigned_by_user_id)
-      )
-
       return unless import_source_user_valid?
 
       Import::ReassignPlaceholderUserRecordsService.new(import_source_user).execute

spec/workers/import/reassign_placeholder_user_records_worker_spec.rb

@@ -51,18 +51,6 @@
         it_behaves_like 'an invalid source user'
       end
 
-      context 'when the feature flag is disabled' do
-        before do
-          stub_feature_flags(importer_user_mapping: false)
-        end
-
-        it 'does not enqueue service to map records to real users' do
-          expect(Import::ReassignPlaceholderUserRecordsService).not_to receive(:new)
-
-          perform_multiple(job_args)
-        end
-      end
-
       it 'queues a DeletePlaceholderUserWorker with the source user ID' do
         expect(Import::DeletePlaceholderUserWorker)
           .to receive(:perform_async).with(import_source_user.id)