Remove underscore hack for IE6

IE6 does not seem to be supported by Bootstrap, and reducing the number
of unused and invalid properties is always nice.

docs/assets/css/bootstrap.css

@@ -1581,7 +1581,6 @@ table .span12 {
   float: left;
   display: none;
   min-width: 160px;
-  _width: 160px;
   padding: 4px 0;
   margin: 0;
   list-style: none;

less/dropdowns.less

@@ -46,7 +46,6 @@
   float: left;
   display: none; // none by default, but block on "open" of the menu
   min-width: 160px;
-  _width: 160px;
   padding: 4px 0;
   margin: 0; // override default ul
   list-style: none;