* Modified hook UserRights::showEditUserGroupsForm to actually be able to do

* Modified hook UserRights::showEditUserGroupsForm to actually be able to do
  something, before it was using $addable and $removable which are not used in
  the showEditUserGroupsForm function since r32610, now it uses $groups instead.
* Changed a $user->getGroups call in showEditUserGroupsForm to use $groups
  instead, the function is passed $groups, which is supposed to be an array of
  groups the user is in, so no point in getting the array again
* Removed a useless line of code in showEditUserGroupsForm
  "list( $addable, $removable ) = $this->splitGroups( $groups );"
  this was never removed from when $addable and $removable were changed to
  just $groups, and so it does nothing and is just confusing.