Remove old and unused .prettyprint styles in code.less (per #10566)

dist/css/bootstrap.css

@@ -737,10 +737,6 @@ pre {
   border-radius: 4px;
 }
 
-pre.prettyprint {
-  margin-bottom: 20px;
-}
-
 pre code {
   padding: 0;
   font-size: inherit;

less/code.less

@@ -33,11 +33,6 @@ pre {
   border: 1px solid @pre-border-color;
   border-radius: @border-radius-base;
 
-  // Make prettyprint styles more spaced out for readability
-  &.prettyprint {
-    margin-bottom: @line-height-computed;
-  }
-
   // Account for some code outputs that place code tags in pre tags
   code {
     padding: 0;