LibWeb: Don't use 'font-weight: lighter' for Csilla in Default.css

Base/res/fonts/CsillaThin7x10.font was renamed to
Base/res/fonts/CsillaRegular10.font in 5abc03d, breaking the default
styles of <code> and <pre>.
The font lookup should still find a font variant when a non-existent
weight is specified, but that's another issue for another day.

Libraries/LibWeb/CSS/Default.css

@@ -30,7 +30,6 @@ h6 {
 
 pre {
     font-family: Csilla;
-    font-weight: lighter;
     margin-bottom: 8px;
     margin-top: 8px;
     white-space: pre;
@@ -38,7 +37,6 @@ pre {
 
 code {
     font-family: Csilla;
-    font-weight: lighter;
 }
 
 u,