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.