Varia: Remove overflow:scroll on pre element

`overflow: scroll` is applied to `pre` elements in our normalize file.
This creates unsightly scrollbars on Verse blocks and others. The latest
version of `normalize.css` doesn't include this particular style.

Let's try removing it, and assuming everything is ok we'll remove it
from Seedlet-based themes too.

Fixes #2842

varia/sass/base/_normalize.scss

@@ -64,7 +64,6 @@ hr {
 pre {
   font-family: monospace, monospace; /* 1 */
   font-size: 1em; /* 2 */
-  overflow: scroll;
 }
 
 /* Text-level semantics

varia/style-rtl.css

@@ -356,7 +356,6 @@ pre {
 	/* 1 */
 	font-size: 1em;
 	/* 2 */
-	overflow: scroll;
 }
 
 /* Text-level semantics

varia/style.css

@@ -356,7 +356,6 @@ pre {
 	/* 1 */
 	font-size: 1em;
 	/* 2 */
-	overflow: scroll;
 }
 
 /* Text-level semantics