Services: Use default constructors/destructors

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Userland/Services/WindowServer/AppletManager.cpp

@@ -25,10 +25,6 @@ AppletManager::AppletManager()
     order_vector = order.split(',');
 }
 
-AppletManager::~AppletManager()
-{
-}
-
 AppletManager& AppletManager::the()
 {
     VERIFY(s_the);