Problem: Do not auto remove user group when new user group was disabled

To find out this bug, we do some step:

• Install new phpbb 3.0.12 system
• Register some user > every user will be in Newly Registered Users group
• Go ACP / General / User registration settings and change New member post limit to 0 to disables "New members are within the Newly Registered Users group until they reach this number of posts" feature
  AND all registered users still are in Newly Registered Users group despite they have posted more new post.