Cleanup remaining "Go Beyond" code

[joshlory]

Followup to PR #15127.

Verified that no GoBeyond levels exist in AWS environments:

Loading production environment (Rails 5.0.1)
irb(main):001:0> GoBeyond.count
=> 0

Loading levelbuilder environment (Rails 5.0.1)
irb(main):001:0> GoBeyond.count
=> 0

Loading staging environment (Rails 5.0.1)
irb(main):001:0> GoBeyond.count
   (4.6ms)  SELECT COUNT(*) FROM `levels` WHERE `levels`.`type` IN ('GoBeyond')
=> 0

[ashercodeorg]

Nice!