Remove bold font-weight property from code snippets (#15067)

In Google Chrome this property is not taken into account, while in
Mozilla Firefox and Safari it is. The property makes the code
snippets difficult to read.

assets/sass/_base.sass

@@ -894,7 +894,6 @@ dd
     font-family: $mono-font
     vertical-align: baseline
     font-size: 14px
-    font-weight: bold
     padding: 2px 4px
 
   a code