Read the Docs is capable of building the PDF version of the BIND 9 ARM using just the contents of the doc/arm/ directory - it does not need the build system to facilitate that. Since the BIND 9 ARM is also built in other formats when "make doc" is run, drop the parts of the build system that enable building the PDF version as they pull in complexity without bringing much added value in return. Update related files accordingly.
I verified that what is left in this branch still allows Read the Docs to build the PDF version of the ARM. We just won't build it ourselves (e.g. in CI or as part of the release process) any more.
Prior Mattermost discussions on this topic that lead to this MR: