Remove pointless db_delete('user_roles') for new users

user_save(), even when inserting a new user, does a db_delete('user_roles') "just to be safe" - this is responsible for 1 of the 13 database queries we issue when saving a new user. Since it's impossible for a user to have any roles when this code executes, there's no reason for it to be there.

The same code, with the same comment, has been in there since 4.6.