LibGfx: Return bool not ErrorOr<bool> from ImageDecoderPlugin::sniff()

Nobody made use of the ErrorOr return value and it just added more
chance of confusion, since it was not clear if failing to sniff an
image should return an error or false. The answer was false, if you
returned Error you'd crash the ImageDecoder.

Userland/Libraries/LibGfx/WebPLoader.cpp

@@ -522,7 +522,7 @@ bool WebPImageDecoderPlugin::initialize()
     return !decode_webp_header(*m_context).is_error();
 }
 
-ErrorOr<bool> WebPImageDecoderPlugin::sniff(ReadonlyBytes data)
+bool WebPImageDecoderPlugin::sniff(ReadonlyBytes data)
 {
     WebPLoadingContext context;
     context.data = data;