Allow people to remove being credited as a mapper

[Exploity]

Feels very abuseable to be able to credit anyone as a mapper on any of your own maps and the other person can't do anything about it

[cl8n]

this sounds good but I also want to point out that people could remove themselves as the mapper even when they actually are supposed to be credited. so maybe lock it in at qualified/ranked/loved

also, if someone is assigning guest mappers in an abusive way, it's probably fair to report them.

[peppy]

Yep, people will be punished if abusing this.