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/FileSystem/FIFO.cpp

@@ -71,9 +71,7 @@ FIFO::FIFO(UserID uid, NonnullOwnPtr<DoubleBuffer> buffer)
     });
 }
 
-FIFO::~FIFO()
-{
-}
+FIFO::~FIFO() = default;
 
 void FIFO::attach(Direction direction)
 {