doc: enable visual code indication in headers

This enables the grey background for inline code in headers.

Refs: https://github.com/nodejs/node/pull/31460#issuecomment-577154740

PR-URL: https://github.com/nodejs/node/pull/31493
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Geoffrey Booth <webmaster@geoffreybooth.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api_assets/style.css

@@ -33,7 +33,6 @@ h3, h3 code,
 h4, h4 code,
 h5, h5 code,
 h6, h6 code {
-  background-color: inherit;
   color: inherit;
   font-family: inherit;
   font-weight: 700;