Stop building release notes as a separate sphinx-doc document

Stop building release notes as a separate sphinx-doc document

The release notes were previously built as a separate document
(including the PDF version).  It was agreed that this doesn't make much
sense, so the release notes are now included only as an appendix to the
BIND 9 ARM.

(cherry picked from commit 8eb2323ec3dee24537b6d8b5b96c2d8c57a0e27f)