Utilities/Release: Remove unused documentation build settings

Since commit 37e015d4 (Utilities/Sphinx: Refactor Sphinx reference
recording, 2023-03-08, v3.27.0-rc1~342^2~1) our documentation requires
Python 3.7 or higher.  This is not available in the base images we use
to build Linux release binaries.  These days our release packaging CI
pipeline builds the documentation in a separate job anyway.