Normalise some pre styles that just look odd

* Remove Liberation Mono from font stack; comes out excessively light on
  hidpi chrome, to the point of unreadability (and probably is not a local
  config problem beyond merely having it installed at all, as chrome
  overrides normal font rendering settings regardless)
* Kill double border on pre styles; doesn't match other such boxes (notice,
  warning, whatever)

Change-Id: I7a95931bd6af4efc880c3ee8f37a91d61f3e6539

resources/screen-common.less

@@ -430,7 +430,6 @@ code {
 pre,
 .mw-code {
 	padding: 1em;
-	border-bottom-width: 0.2em;
 	/* Wrap lines in overflow. T2260, T103780 */
 	white-space: pre-wrap;
 }

resources/themes/wikimedia.less

@@ -34,7 +34,7 @@
 // Fonts are chosen for consistent metrics, not necessarily overall prettiness.
 @fonts: 'Segoe UI', 'Segoe UI Emoji', 'Segoe UI Symbol', 'Lato', 'Liberation Sans', 'Noto Sans', 'Helvetica Neue', 'Helvetica', sans-serif;
 @fonts-secondary: 'Linux Libertine', 'Times New Roman', 'Liberation Serif', 'Nimbus Roman', 'Noto Serif', 'Times', serif;
-@fonts-mono: 'Liberation Mono', 'Menlo', 'Consolas', 'Monaco', 'Noto Mono', 'Nimbus Mono L', monospace;
+@fonts-mono: 'Menlo', 'Consolas', 'Monaco', 'Noto Mono', 'Nimbus Mono L', monospace;
 
 @border: 0.2em;
 @radius: 0.2em;