Simplify augmented module for express-flash

There is existing @types definition for express-flash. Let's use it and
just augment a method required by the existing code to please compiler.

Thanks!

src/types/express-flash.d.ts

@@ -1,9 +1,13 @@
-
 /// <reference types="express" />
 
-// Add RequestValidation Interface on to Express's Request Interface.
+/**
+ * This type definition augments existing definition
+ * from @types/express-flash
+ */
 declare namespace Express {
-    interface Request extends Flash {}
+    export interface Request {
+        flash(event: string, message: any): any;
+    }
 }
 
 interface Flash {