LibGfx: Remove Gfx::ImageDecoder::bitmap() in favor of frame(index)

To transparently support multi-frame images, all decoder plugins have
already been updated to return their only bitmap for frame(0).

This patch completes the remaining cleanup work by removing the
ImageDecoder::bitmap() API and having all clients call frame() instead.

Userland/Libraries/LibGfx/PNGLoader.cpp

@@ -1013,10 +1013,9 @@ size_t PNGImageDecoderPlugin::frame_count()
 
 ImageFrameDescriptor PNGImageDecoderPlugin::frame(size_t i)
 {
-    if (i > 0) {
-        return { bitmap(), 0 };
-    }
-    return {};
+    if (i > 0)
+        return {};
+    return { bitmap(), 0 };
 }
 
 }