Since I am exclusively using `topen`
(https://git.martyoeh.me/Marty/topen) for my task notes currently, this
commit gets rid of any left-over config files setting up `taskopen`.
Starts to simplify the taskwarrior setup a tiny bit.
Since I am striving for closer connection between my task management, my
mail suite and my calendar appointments, it makes sense to unify all
'office' tasks into the single office module instead of keeping
taskwarrior outside of it.
