LibJS: Remove reduntant set_prototype() calls

If we're inheriting from Object() we already call its constructor which
sets the prototype - no need to do it again.

Libraries/LibJS/Runtime/Date.cpp

@@ -42,7 +42,6 @@ Date::Date(Core::DateTime datetime, u16 milliseconds, Object& prototype)
     , m_datetime(datetime)
     , m_milliseconds(milliseconds)
 {
-    set_prototype(&prototype);
 }
 
 Date::~Date()