added remove-from-group action

[wellenreiter01]

It think it is a usefull enhancement have the possibility to remove users from a group based on their reputatation or number of posts.

Possible usage:

• remove user from newbies group
• restrict users with negative reputation (= spammer?) to given groups

[psychobunny]

Thanks! Sorry for the late merge. The code was missing this: https://github.com/NodeBB/nodebb-rewards-essentials/commit/6d0eec9c2b5c4c5edca34346cb999e7d69488908

works perfectly now! :)