Remove placeholder in SDG related list

Its value is difficult to read due to the low contrast between the color
of the field and the color of the placeholder text, and we already have
the same information in the hint above it.

app/assets/javascripts/sdg/related_list_selector.js

@@ -89,6 +89,12 @@
       suggestions_input[0].id = original_input[0].id + "_suggestions";
 
       $("[for='" + original_input[0].id + "']").attr("for", suggestions_input[0].id);
+
+      suggestions_input.attr("aria-describedby", original_input.attr("aria-describedby"));
+
+      if ($(original_input).attr("placeholder") === undefined) {
+        suggestions_input.removeAttr("placeholder");
+      }
     }
   };
 }).call(this);