Command to remove start time for a specific day

[gustav-langer]

Hello,
when using this bot today, I accidentally typed !start 1 instead of !start 2, so now a negative finish time is stored for that day, messing up the custom leaderboard.

For situations like this, it would be great to have a command like !remove <DAY_NUMBER> to delete an entry from the database.

(Additionally, the code could also check that a challenge is not already completed before storing the start time and fail if it is.)