Fixed code block line-number bar showing in exports

Also fixed in print view.
Likely crept in during CM6 changes.

For #4215

Author: ssddanbrown

resources/sass/_text.scss

@@ -202,7 +202,7 @@ pre {
   pre {
     padding-left: 12px;
   }
-  pre:after {
+  pre:before {
     display: none;
   }
 }

resources/sass/export-styles.scss

@@ -33,7 +33,7 @@ table {
 pre {
   padding-left: 12px;
 }
-pre:after {
+pre:before {
   display: none;
 }
 pre code {