LibGfx/PNG: Decode the header in create() and remove initialize()

This is done as a part of #19893. As `create()` is now the last user of
`decode_png_ihdr()`, we can easily make it return an `ErrorOr`.

Userland/Libraries/LibGfx/ImageFormats/PNGLoader.h

@@ -21,7 +21,6 @@ public:
 
     virtual IntSize size() override;
 
-    virtual ErrorOr<void> initialize() override;
     virtual bool is_animated() override;
     virtual size_t loop_count() override;
     virtual size_t frame_count() override;