Automatically remove wait date when completing a task

[xeruf]

Sometimes, I complete a task right off my someday-list. In that case, the wait date should be automatically deleted - keeping it doesn't make much sense.

[skbolton]

This is a good use case for a hook if this is the behavior you want.

[tbabej]

I think @xerus2000 is right about this one. At first I was considering we might want to keep it for historical purposes, but the information would be preserved by the change history.

[tbabej]

Reproduced on 2.6.0.

[mrossinek]

I opened #2371 to implement this 👍

[tbabej]

Implemented by @mrossinek.