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/PNGWriter.h

@@ -19,7 +19,7 @@ public:
     static ByteBuffer encode(Gfx::Bitmap const&);
 
 private:
-    PNGWriter() { }
+    PNGWriter() = default;
 
     Vector<u8> m_data;
     void add_chunk(PNGChunk&);