Remove leftover duplicate code
Remove leftover duplicate code

When splitting the codecs into dec/enc, it seems that the decoding code
was not removed from the enc part of codec_jpg.

libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl