Better styling for options_element widget

The file webform-admin.css contains a piece of styling for the options_element widget. It styles three elements:

.webform-options-element thead {
  display: none;
}
.webform-options-element fieldset {
  border: none;
  background: none;
  margin: 0;
  padding: 0;
}
.webform-options-element fieldset legend {
  display: none;
}

I opt to remove those rules because:

• .webform-options-element thead is a very useful table heading incidicating what field is the key, what field is the value and what the radio button does
• .webform-options-element fieldset matches no element, because it should be fieldset.webform-options-element if we'd want to hide the legend, which I think we don't.

Attached patch removes the css rules. Feedback welcome!