Remove deckar01-task_list

What does this MR do and why?

Remove deckar01-task_list — fixes Remove deckar01-task_list gem dependency (#581837).

Note there is a (necessary) functionality change here: system notes for task item status changes ("marked the checklist item XYZ as completed" and "as incomplete") no longer contain formatting from the task item.

We used to include the Markdown from the entire original line of input right here, allowing user formatting to become permanently part of the system note. You could also trivially break formatting in system notes this way, with unpredictable and occasionally security-breaking) results.

How to set up and validate locally

1. Check out the branch, run bundle install and restart your GDK.

2. Test out toggling task items in issues/MRs/notes.

3. Verify that everything looks OK, system notes are generated successfully, and the task completion status is reported correctly in other places; this example shows task item completion on the MR list, which relies on this code for its data:

   

MR acceptance checklist

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