Removed a duplicate rule

src/notebook/index.css

@@ -81,13 +81,6 @@
 }
 
 
-.jp-MarkdownCell-content.p-Widget h4 {
-  font-family: "Helvetica Neue", Arial, Helvetica, Geneva, sans-serif;
-  font-size: 14px;
-  line-height: 1.0;
-  margin: 14px 0 0 0;
-}
-
 
 .jp-MarkdownCell-content.p-Widget h5 {
   font-family: "Helvetica Neue", Arial, Helvetica, Geneva, sans-serif;