[BB-5478] Remove "clean sprints feedback" feature

When a team member completes a sprint without spillovers, a Jira bot pings them on the cell "Meetings" ticket, asking about sharing some tips for other team members about time management.

We've introduced this feature in September 2019, and since then it has been used 17 times (last time in July 2020).

We already have:

• A great handbook section about time management.
• Sprint retrospectives, which we're publishing on the forum.
• Checkup forms, submitted every sprint.

Therefore, it looks like a good time to stop maintaining this feature through the planned reworks.