[panel] Remove vertical padding for panel-button

This fixes the status menu requesting 27 pixels, which causes
the slicer for the icon to overdraw.

https://bugzilla.gnome.org/show_bug.cgi?id=616951

data/theme/gnome-shell.css

@@ -119,7 +119,7 @@ StTooltip {
 }
 
 .panel-button {
-    padding: 4px 12px 3px;
+    padding: 0px 12px;
     border-radius: 5px;
     border-radius-bottomleft: 0px;
     border-radius-bottomright: 0px;