This reverts commit 0878ca7ab4c9da80ba18357cfd29596dc0500973.
My change to enable JBIG for splix did not work correctly. I was unable to get to the bottom of it because the build never completed. Given that it's also timing out in Hydra (see https://hydra.nixos.org/eval/1372226) I think it's best to simply back it out.