css: Keep the panel height the same in all modes

Since the panel affects struts, if we change its height on mode
transitions, windows will move around which is particularly noticable
with maximized windows when coming out of the screen shield.

https://bugzilla.gnome.org/show_bug.cgi?id=692966

data/theme/gnome-shell.css

@@ -432,13 +432,11 @@ StScrollBar StButton#vhandle:active {
 }
 
 #panel.lock-screen {
-    height: 2em;
     background-color: rgba(0,0,0,0.3);
 }
 
 #panel.unlock-screen,
 #panel.login-screen {
-    height: 2em;
     background-color: transparent;
 }