web_service: Remove unnecessary inclusions

Reduces the potential amount of rebuilding necessary if any headers
change. In particular, we were including a header from the core library
when we don't even link the core library to the web_service library, so
this also gets rid of an indirect dependency.

src/web_service/verify_login.h

@@ -4,8 +4,6 @@
 
 #pragma once
 
-#include <functional>
-#include <future>
 #include <string>
 
 namespace WebService {