Libraries: Use default constructors/destructors in LibGfx

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/LibGfx/ImageDecoder.cpp

@@ -79,8 +79,4 @@ ImageDecoder::ImageDecoder(NonnullOwnPtr<ImageDecoderPlugin> plugin)
 {
 }
 
-ImageDecoder::~ImageDecoder()
-{
-}
-
 }