Code refactor: Remove unused selectors from Bartik's table.css

@media screen and (max-width: 37.5em) { /* 600px */
  th.priority-low,
  td.priority-low,
  th.priority-medium,
  td.priority-medium {
    display: none;
  }
}
@media screen and (max-width: 60em) { /* 920px */
  th.priority-low,
  td.priority-low {
    display: none;
  }
}

.priority-low always gets overridden by the next css rule.