Libraries: Use default constructors/destructors in LibCore

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/Libraries/LibCore/CMakeLists.txt

@@ -20,7 +20,6 @@ set(SOURCES
     MappedFile.cpp
     MimeData.cpp
     NetworkJob.cpp
-    NetworkResponse.cpp
     Notifier.cpp
     Object.cpp
     Process.cpp