Kernel: 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."

Kernel/Memory/PageDirectory.cpp

@@ -123,9 +123,7 @@ ErrorOr<NonnullRefPtr<PageDirectory>> PageDirectory::try_create_for_userspace(Vi
     return directory;
 }
 
-PageDirectory::PageDirectory()
-{
-}
+PageDirectory::PageDirectory() = default;
 
 UNMAP_AFTER_INIT void PageDirectory::allocate_kernel_directory()
 {