decoder: removed unused destructor

Very old unused code.

app/codec/decoder.cpp

@@ -47,10 +47,6 @@ Decoder::Decoder(Stream *fs) :
 {
 }
 
-Decoder::~Decoder()
-{
-}
-
 StreamPtr Decoder::stream()
 {
   return stream_;

app/codec/decoder.h

@@ -70,9 +70,6 @@ public:
 
   Decoder(Stream* fs);
 
-  // Necessary for subclassing, it's empty
-  virtual ~Decoder() override;
-
   DISABLE_COPY_MOVE(Decoder)
 
   virtual QString id() = 0;