content [nfc]: Stop redundantly setting spans' background in code blocks

The code block background is currently white anyway, so this has
been redundant. But we're about to set it to something off-white, to
align with web, and this white background color on the text spans
would interfere with that.

Author: chrisbobbe

lib/widgets/content.dart

@@ -858,9 +858,7 @@ final _kInlineCodeStyle = kMonospaceTextStyle
   .merge(const TextStyle(backgroundColor: Color(0xffeeeeee)));
 
 final _kCodeBlockStyle = kMonospaceTextStyle
-  .merge(const TextStyle(
-    backgroundColor: Color.fromRGBO(255, 255, 255, 1),
-    fontSize: 0.825 * kBaseFontSize))
+  .merge(const TextStyle(fontSize: 0.825 * kBaseFontSize))
   .merge(
     // TODO(a11y) pass a BuildContext, to handle platform request for bold text.
     //   To get one, the result of this whole computation (to the TextStyle