[panel] Remove border bottom

This line just looks poor - it's probably a leftover from when the
panel was a gradient.  We didn't quite see it before because of
a different bug where we were over-allocating the panel to 27
pixels.

data/theme/gnome-shell.css

@@ -99,7 +99,6 @@ StTooltip {
     color: #ffffff;
     font-size: 16px;
     background-color: black;
-    border-bottom: 1px solid #1f1f1f;
 }
 
 #panelLeft, #panelCenter, #panelRight {