Revert "Move the popup close box sizing to CSS"

Moving the box size to CSS broke `Popup.setSize()`; the function
expects a width for the padding computation. Because this function is
called before the div is added to the DOM, the size can't be
computed.

This reverts commit 134f24480a.

theme/default/style.css

@@ -259,8 +259,6 @@ div.olControlSaveFeaturesItemInactive {
 
 .olPopupCloseBox {
   background: url("img/close.gif") no-repeat;
-  width: 17px;
-  height: 17px;
   cursor: pointer;
 }