Reuse `Gitlab::ShellAdapter`

lib/gitlab/background_migration/update_authorized_keys_file_since.rb

 module Gitlab
   module BackgroundMigration
     class UpdateAuthorizedKeysFileSince
+      include Gitlab::ShellAdapter
+
       class Key < ActiveRecord::Base
         self.table_name = 'keys'
 
@@ -34,10 +36,6 @@ module Gitlab
           end
         end
       end
-
-      def gitlab_shell
-        @gitlab_shell ||= Gitlab::Shell.new
-      end
     end
   end
 end

spec/lib/gitlab/background_migration/update_authorized_keys_file_since_spec.rb

@@ -28,7 +28,7 @@ describe Gitlab::BackgroundMigration::UpdateAuthorizedKeysFileSince do
     end
 
     it 'calls remove_keys_not_found_in_db on Gitlab::Shell' do
-      expect(background_migration.gitlab_shell).to receive(:remove_keys_not_found_in_db)
+      expect_any_instance_of(Gitlab::Shell).to receive(:remove_keys_not_found_in_db)
       subject
     end
   end
@@ -67,7 +67,7 @@ describe Gitlab::BackgroundMigration::UpdateAuthorizedKeysFileSince do
 
   describe '#remove_keys_not_found_in_db' do
     it 'calls remove_keys_not_found_in_db on Gitlab::Shell' do
-      expect(background_migration.gitlab_shell).to receive(:remove_keys_not_found_in_db)
+      expect_any_instance_of(Gitlab::Shell).to receive(:remove_keys_not_found_in_db)
       background_migration.remove_keys_not_found_in_db
     end
   end