Per John Cupitt, these are not necessary

#41 (comment)

John Cuppit (libvips creator and maintainer) found issues with the
heroku-built libheif and the compiled version of pdfium. He recommended
switching back to poppler as a result.

#41 (comment)
#41 (comment)

The stacks are getting more and more native libraries that we depend on
installed by Heroku. This allows us to remove them from our build and
just include the -dev packages that match the runtime packages that
Heroku installed.

One of the drivers of Rails switching to libvips was the security risks
posed by imagemagick. It would be better if we don't add that risk back
to Rails apps by including it in the libvips build.