FIX: Users should be able to remove their primary group

This fix allows a user to remove their currently assigned primary group
if the Site Setting `user selected primary groups` is enabled.

Before this fix, if a user selected "none" for their primary group it
would silently fail and never be updated.

app/services/user_updater.rb

@@ -89,6 +89,10 @@ class UserUpdater
       guardian.can_use_primary_group?(user, attributes[:primary_group_id])
 
       user.primary_group_id = attributes[:primary_group_id]
+    elsif SiteSetting.user_selected_primary_groups &&
+      !attributes[:primary_group_id].present?
+
+      user.primary_group_id = nil
     end
 
     CATEGORY_IDS.each do |attribute, level|