css: Remove redundant declaration from night mode CSS.

This declaration already exists in the default CSS.

This declaration was present when the edit history modal was first
given a night mode (then called "dark mode") style in November 2017 in
4f81bdd0a6. It also existed in the
default CSS at that time.

static/styles/night_mode.scss

@@ -491,7 +491,6 @@ on a dark background, and don't change the dark labels dark either. */
             .highlight_text_deleted {
                 color: hsl(0, 90%, 67%);
                 background-color: hsla(7, 54%, 62%, 0.38);
-                text-decoration: line-through;
             }
         }
     }