Remove hide completed code form taskClasses.

My PR (habitrpg/habitrpg#3574) to habitrpg/habitrpg refactors this out of taskClasses, so this is no longer needed.

## EDIT BY ALYS: DO NOT MERGE THIS PR. See the comments in https://github.com/HabitRPG/habitrpg/pull/3574. I'll close this PR once all the related work is finished.