combobox: Don't special-case RTL child positions anymore

combobox: Don't special-case RTL child positions anymore

If you want to get rounded corners on an hbox, instead of
  :first-child {
    border-top-left-radius: 5px;
    border-bottom-left-radius: 5px;
  }
  :last-child {
    border-top-right-radius: 5px;
    border-bottom-right-radius: 5px;
  }
you now need to write:
  :first-child, :last-child:dir(rtl) {
    border-top-left-radius: 5px;
    border-bottom-left-radius: 5px;
  }
  :last-child, :first-child:dir(rtl)
  {
    border-top-right-radius: 5px;
    border-bottom-right-radius: 5px;
  }