don't split `.cell` and `div.cell` CSS

`.cell` matches non-IPython things that it shouldn't (inside highlighted code),
everything `.cell` should match in IPython is a div.

IPython/html/static/notebook/less/cell.less

@@ -1,4 +1,4 @@
-.cell {
+div.cell {
     border: 1px solid transparent;
     .vbox();
 
@@ -6,9 +6,6 @@
         .corner-all;
         border : thin @border_color solid;
     }
-}
-
-div.cell {
     width: 100%;
     padding: 5px 5px 5px 0px;
     /* This acts as a spacer between cells, that is outside the border */