Enable the no-else-return ESLint rule

Using `else` after `return` is not necessary, and can often lead to unnecessarily cluttered code. By using the `no-else-return` rule in ESLint we can avoid this pattern, see http://eslint.org/docs/rules/no-else-return.

web/pdf_history.js

@@ -284,9 +284,8 @@
           this.nextHashParam = null;
           this.updatePreviousBookmark = true;
           return;
-        } else {
-          this.nextHashParam = null;
         }
+        this.nextHashParam = null;
       }
 
       if (params.hash) {